Chapter 1. Introduction to Groovy

We can’t talk much about Grails without a solid understanding of Groovy, because it’s so integral to how Grails works.

Groovy is a JVM language with a primary goal of extending Java. By adding a Meta Object Protocol (MOP) to enable metaprogramming, Groovy adds powerful capabilities that enable dynamic programming (changing and adding behavior at runtime), domain-specific languages (DSLs), and a huge number of convenience methods and approaches that simplify your code and make it more powerful.

Groovy compiles to bytecode just like Java does (although it creates different bytecode). As Java developers, we tend to think that only javac can compile source code to create .class files, but there are many JVM languages that do as well (including Groovy, JRuby, Jython, and hundreds more). There are also libraries such as BCEL that you can use to programmatically create bytecode. As a result, Groovy and Java interoperate well; you can call Java methods from Groovy and vice versa, a Java class can extend a Groovy class or implement a Groovy interface, and in general, you don’t need to even think about interoperability because it “just works.”

The Groovy equivalent of javac is groovyc, and because it compiles both Groovy and Java code, it’s simple to use for your project code. Of course in Grails applications, we rarely even think about this process (except when it fails) because Grails scripts handle that, but if you’re manually compiling code (e.g., in a Gradle build or with Gant), it’s about as simple as working with Java code.

Installing Groovy

Ordinarily, Grails developers don’t install a Groovy distribution, because each version of Grails ships with the groovy-all JAR with which it was developed. Groovy is a fundamental part of Grails, so using it in a Grails application is trivial. But it’s easy to install if you want to use Groovy outside of Grails, for example, for scripting or to run standalone utility applications. Just download the version you want, unpack the ZIP file to your desired location, set the GROOVY_HOME environment variable point at the location you chose, and add the $GROOVY_HOME/bin (or %GROOVY_HOME%\bin in Windows) directory to the PATH environment variable. That’s all you need to do; run groovy -v from a command prompt to verify that everything is working.

If you’re using Windows, the download page has installers that will install the distribution and configure the environment.

There is also a new tool, GVM (Groovy enVironment Manager). It was inspired by the Ruby RVM and rbenv tools, and it will install one or more versions of Groovy, as well as Grails, Griffon, Gradle, and vert.x. It uses the bash shell, so it works in Linux, OS X, and Windows, if you have Cygwin installed. It’s very simple to use, and if you have projects that require different versions of Groovy, it’s easy to switch between them. See the GVM site for usage information.

Groovy Console

The Groovy console is a great way to prototype code. It doesn’t have many text editor or IDE features, but you can run arbitrary Groovy code and inspect the results. You can run it in debug mode and attach to it from a debugger (e.g., your IDE) to dig deeper and look at the call stack. It’s convenient to test an algorithm or a fix, or to do what-if experiments. And you don’t need to create a class or a main() method—you can execute any valid code snippet. If Groovy is installed and in your PATH, run the console by executing groovyConsole from the command line. I encourage you to test out the code examples as they’re shown to make sure you understand how everything works.

The Groovy console is also a part of Grails—you can run grails console from the command line and start the Grails version of the console. It’s the same application, but it also has Grails-specific hooks like easy access to the Spring ApplicationContext and automatic application of the PersistenceContextInterceptors. You can use it to call Grails object relational mapping (GORM) methods, services, and pretty much anything in your application that isn’t related to an HTTP request. As a plugin author, I often troubleshoot bean definition issues by running the following (as shown in Figure 1-1):

ctx.beanDefinitionNames.sort().each { println it }

This grabs all of the Spring bean names (a String[] array) from the ApplicationContext (the ctx binding variable), sorts them (into a new List), and prints each name. The true statement at the end is a trick to avoid printing the entire list again in its toString() form, because the console treats the last statement as the return value of the script and renders it in the output window.

Grails console
Figure 1-1. Grails console

Optional Typing

One of Groovy’s strengths comes from its support of optional typing. You can define the types of variables, method parameters, method return values, and so on, like you do in Java, but you often don’t need to. Groovy determines the actual type at runtime and invokes the methods on the objects if they exist (or if you’ve added support to the metaclass; more on this later). The approach used is often called duck typing; i.e., if it walks and talks like a duck, consider it a duck.

This isn’t the same as weak typing. The objects themselves have a concrete type (unlike JavaScript, C, Perl, and so on), but you’re not restricted by the compiler to only invoke methods defined in the specified type of the object. If the object supports the call, it will work.

In fact, you’re not even restricted to hardcoding the method or property names. You can dynamically invoke a method or access a property value by name:

def person = ...

String methodName = ...
def value = person."$methodName"(1, 2)

String propertyName = ...
def otherValue = person."$propertyName"

Collections and Maps

Creating and populating Java collections might not seem that bad if you haven’t seen how it’s done in Groovy, but once you have, you won’t want to go back. Here’s some code to add a few elements to an ArrayList in Java:

List<String> things = new ArrayList<String>();

And here’s the equivalent code in Groovy:

List<String> things = ["Hello", "Groovy", "World"]

The difference is rather stark, and using more idiomatic Groovy (there’s not much need for generics in Groovy), it’s even cleaner:

def things = ['Hello', 'Groovy', 'World']

Note that here I’m taking advantage of Groovy’s support for declaring strings using single or double quotes; this is described in more detail later in the chapter.

There isn’t a separate syntax for a Set, but you can use type coercion for that. Either:

Set things = ['Hello', 'Groovy', 'World']


def things = ['Hello', 'Groovy', 'World'] as Set

The syntax for a Map is similar, although a bit larger, because we need to be able to specify keys and values delimited with colons:

def colors = ['red': 1, 'green': 2, 'blue': 3]

We can make that even more compact because, when using strings as keys that have no spaces, we can omit the quotes:

def colors = [red: 1, green: 2, blue: 3]

You might be wondering what the type of these collections is—some funky Groovy-specific interface implementations that handle all the details of the Groovy magic happening under the hood? Nope, Lists and Sets are just regular java.util.ArrayList and java.util.HashSet instances. Maps are java.util.LinkedHashMap instances instead of the more common java.util.HashMap; this is a convenience feature that maintains the order in the map based on the declaration order. If you need the features of other implementations such as LinkedList or TreeMap, just create them explicitly like you do in Java.

Lists and Maps support array-like subscript notation:

def things = ['Hello', 'Groovy', 'World']
assert things[1] == 'Groovy'
assert things[-1] == 'World'

def colors = [red: 1, green: 2, blue: 3]
assert colors['red'] == 1

Maps go further and let you access a value using a key directly as long as there are no spaces:

def colors = [red: 1, green: 2, blue: 3]
assert == 2


You’ve heard of POJOs—Plain Old Java Objects—and JavaBeans. These are simple classes without a lot of extra functionality and, in the case of JavaBeans, they follow conventions such as having a zero-argument constructor and having getters and setters for their attributes. In Groovy, we create POGOs—Plain Old Groovy Objects—that are analogous and work the same way, although they’re more compact.

Consider a POJO that represents a person in your application. People have names, so this Person class should have firstName, initial, and lastName attributes to store the person’s full name. In Java, we represent those as private String fields with getter methods, and setter methods if we’re allowing the attributes to be mutable. But often we don’t do any work when setting or getting these values—we just store them and retrieve them. But dropping this encapsulation and replacing each private field, getter, and setter with a public field would be limiting in the future because, at some point, there might be a reason to manipulate the value before storing or retrieving it. So we end up with a lot of repetetive boilerplate in these POJOs. Sure, our IDEs and other tools can autogenerate the code and we can ignore it and pretend that it’s not there, but it is, and it unnecessarily bulks up our codebase.

Groovy fixes this mess for us by automatically generating getters and setters for public properties during compilation. But that’s only the case if they’re not already there; so this gives you the flexibility of defining attributes as public fields while retaining the ability to override the behavior when setting or getting the values. Groovy converts the public field to a private field but pretends the public field is still there. When you read the value, it calls the getter; and when you set the value, it calls the setter.

Consider this POGO:

class Thing {
   String name
   int count

The default scope for classes, fields, and methods is public (more on this later), so this is a public class and the two fields are public. The compiler, however, will convert these to private fields and add getName(), setName(), getCount(), and setCount() methods. This is most clear if you access this class from Java; if you try to access the name or count fields, your code won’t compile.

Although Groovy generates getters and setters for you, you can define your own:

class Thing {
   String name
   int count

   void setName(String name) {
      // do some work before setting the value = name
      // do some work after setting the value

and in this case, only the setName(), getCount(), and setCount() methods will be added.

You can also have read-only and write-only properties. You can create an immutable read-only property by making the field final and setting it in a parameterized constructor:

class Thing {
   final String name
   int count

   Thing(String name) { = name

Because it’s final, the compiler doesn’t even generate a setter method, so it cannot be updated. If you want to retain the ability to update it internally, make the field private and create a getter method. Because it’s private, the compiler won’t generate the setter:

class Thing {
   private String name
   int count

   String getName() { name }

You’ll need a parameterized constructor to set the value, or set it in another method. Creating a write-only property is similar; use a private field and create only the setter:

class Thing {
   private String name
   int count

   void setName(String name) { = name }


In general, it is safe to replace getter and setter method calls with property access; for example, person.firstName is a lot more compact than person.getFirstName() and equivalent. One case where it’s not safe is with the getClass() method and Maps. If you try to determine the class of a Map instance using the .class property form of the getClass() method, Groovy will look up the value stored under the "class" key and probably return null. I always use getClass() even when I know the object isn’t a Map just to be on the safe side.

Using the AST Browser

During compilation, Groovy represents your code in memory as an Abstract Syntax Tree (AST). The Groovy console’s AST browser is one way to see what is going on under the hood. There are several compilation phases (parsing, conversion, semantic analysis, and so on), and the AST browser will show you graphically what the structure looks like at each phase. This can help to diagnose issues, and is particularly helpful when you write your own AST transformations, where you can hook into the bytecode generation process and add your own programmatically. Figure 1-2 shows the state at the Class Generation phase.

AST browser
Figure 1-2. AST browser

Decompiling with JD-GUI

I highly recommend decompiling Groovy .class files to get a sense of what is added during the compilation process. It’s one thing to believe that getters and setters are added for you, but it’s another to actually see them. And there can be a lot of generated code in some of your classes; for example (jumping ahead a bit here), Grails uses several AST transformations to add compile-time metaprogramming methods to controllers, domain classes, and other artifacts. JD-GUI is an excellent free decompiler that I’ve had a lot of success with. Figure 1-3 shows an example class.

Figure 1-3. JD-GUI

Decompiling with javap

Another option that doesn’t require third-party software is javap, which is part of the JDK install. Running it with no switches will display the method signatures, e.g., javap target/classes/, and passing the -c switch will decompile the code into a readable form of the bytecode, not the Groovy or analogous Java source; e.g., javap -c target/classes/ The output isn’t anywhere near as readable as what you get with a decompiler like JD-GUI, but it can be more convenient for a quick look.


Closures are an important aspect of Groovy. As a Grails developer you’ll use them a lot; they define controller actions (although in 2.0, methods are supported and are preferred) and taglib tags and are used to implement the constraints and mapping blocks in domain classes, the init and destroy blocks in BootStrap.groovy, and in fact most of the blocks in the configuration classes in grails-app/conf. They also provide the functionality that makes builders and DSLs so powerful. But what are they?

A closure is a block of code enclosed in braces. Closures are similar to function pointers in C and C++ in that you can assign them to a variable and pass them as method parameters and invoke them inside the methods. They’re also similar to anonymous inner classes, although they don’t implement an interface or (at least explicitly) extend a base class (but they can be used to implement interfaces—more on that later).

A closure can be as simple as:

def hello = { println "hello" }

A closure can be invoked by calling its call method:

def hello = { println "hello" }

but Groovy lets you use a more natural method call syntax (it invokes the call method for you):

def hello = { println "hello" }

Like methods, closures can have parameters, and there are three variants. In the hello example, because there’s nothing declared, there is one parameter with the default name it. So a modified closure that prints what it’s sent would be:

def printTheParam = { println it }

and you could call it like this:


You can omit parentheses like you can with method calls:

printTheParam 'hello'

Named arguments use -> to delimit the parameters from the code:

def printTheParam = { whatToPrint -> println whatToPrint }

and, like method arguments, they can be typed:

def add = { int x, int y -> x + y }

If the closure has no arguments, use the -> delimiter and the it parameter will not be available:

def printCurrentDate = { -> println new Date() }

You can determine the number of parameters that a closure accepts with the getMaximumNumberOfParameters() method and get the parameter types (a Class[] array) with getParameterTypes().

A closure is a subclass of groovy.lang.Closure that is generated by the Groovy compiler; you can see this by running:

println hello.getClass()

The class itself will have a name like ConsoleScript14$_run_closure1. Nested closures extend this naming convention; for example, if you look in the classes directory of a Grails application, you’ll see names like BuildConfig$_run_closure1_closure2.class, which are the result of having repositories, dependencies, and plugins closures defined within the top-level grails.project.dependency.resolution closure.

The dollar sign in the class and filename will look familiar if you’ve used anonymous inner classes before. In fact, that’s how they’re implemented. They’re different from anonymous inner classes in that they can access nonfinal variables outside of their scope. This is the “close” part of closure—they enclose their scope, making all of the variables in the scope the closure is in available inside the closure. This can be emulated by an inner class by using a final variable with mutable state, although it’s cumbersome. For example, this Java code doesn’t compile, because i isn’t final, and making it final defeats the purpose, because it needs to be changed inside the onClick method:

interface Clickable {
   void onClick()

int i = 0;
Clickable c = new Clickable() {
   public void onClick() {
      System.out.println("i: " + i);

We can fix it with a final 1-element array (because the array values are still mutable):

final int[] i = { 0 };
Clickable c = new Clickable() {
   public void onClick() {
      System.out.println("i: " + i[0]);

but it’s an unnatural coding approach. The Groovy equivalent with a closure is a lot cleaner:

int i = 0
def c = { ->
   println "i: $i"
} as Clickable

So how does Groovy break this JVM rule that anonymous inner classes can’t access nonfinal variables? It doesn’t—it uses a trick like the one above. Instead of using arrays like the above example, there’s a holder class, groovy.lang.Reference. Enclosed variable values are stored in final Reference instances, and Groovy transparently makes the values available for you. The only time you’ll see this occurring is when you’re stepping through code in debug mode in an IDE.

Interface Coercion

The previous example demonstrates interface coercion; because the Clickable interface has only one method, the closure can implement that method if it has the same parameter type(s). The as keyword tells the Groovy compiler to create a JDK dynamic proxy implementing the interface. This is the simple version, but it also works for interfaces with multiple methods.

To implement an interface with more than one method, create a Map with method names as keys and closures with the corresponding parameter types as values:

import java.sql.Connection

def conn = [
   close: { -> println "closed" },
   setAutoCommit: { boolean autoCommit -> println "autocommit: $autoCommit" }
] as Connection

One useful aspect of this approach is that you aren’t required to implement every method. Calling close or setAutoCommit will invoke the associated closures as if they were methods, but calling an unimplemented method (e.g., createStatement()) will throw an UnsupportedOperationException. This technique was more common before anonymous inner class support was added to Groovy in version 1.7, but it’s still very useful for creating mock objects when testing. You can implement just the methods that will be called and configure them to work appropriately for the test environment (e.g., to avoid making a remote call or doing database access) and switch out the mock implementation in place of the real one.

Programmatic Closures

Although it’s rare to do so, you can create a closure programmatically (most likely from Java). You might do this if you have some reason to implement some code in Java but need to pass a closure as a parameter to a Groovy method. The Closure class is abstract but doesn’t have any abstract methods. Instead, you declare one or more doCall methods with the supported call signatures:

Closure<String> closure = new Closure<String>(this, this) {
   public String doCall(int x) {
      return String.valueOf(x);
   public String doCall(int x, int y) {
      return String.valueOf(x * y);

This can be invoked from Groovy just like one created the typical way:

closure(6) // prints "6"
closure(6, 2) // prints "12"
closure(1, 2, 3) // throws a groovy.lang.MissingMethodException
                 // since there's no 3-param doCall()

Owner, Delegate, and this

this inside a closure is probably not what you expect. Intuitively, it seems like it should be the closure itself, but it turns out that it’s actually the class instance where the closure is defined. As such, it’s probably not of much use—the owner and delegate are much more useful.

The owner of a closure is the surrounding object that contains the closure. It functions as the target of method invocations inside the closure, and if the method isn’t defined, then a MissingMethodException will be thrown. In this example, if we create a new class instance and call the callClosure method (new SomeClass().callClosure()), it will print Woof!, because the dogAndCat closure calls the existing woof method, but it will then fail on the meow call because it doesn’t exist:

class SomeClass {

   void callClosure() {

      def dogAndCat = {


   void woof() {
      println "Woof!"

You can assign a delegate for a closure to handle method calls. By default, the delegate is the owner, but you can change it with the setDelegate method. This is frequently used when parsing DSLs. The DSL can be implemented as a closure, and inner method calls and property access can be routed to a helper (i.e., the DSL builder), which implements the logic required when a method or property is called that’s not locally defined but is valid in the DSL.

One example is the mapping block in Grails domain classes. This is a static closure that, if defined, will be used to customize how the class and fields map to the database:

class User {
   String username
   String password

   static mapping = {
      version false
      table 'users'
      password column: 'passwd'

If you were to invoke the mapping closure (User.mapping()), you would get a MissingMethodException for each of the three lines in the closure, because the owner of the closure is the User class and there’s no version, table, or password methods (and none added to the metaclass). It’s more clear that these are method calls if we add in the optional parentheses that were omitted:

static mapping = {
   password(column: 'passwd')

Now we see that it’s expected that there’s a version method that takes a boolean parameter, a table method that takes a String, and a password method that takes a Map. Grails sets the delegate of the closure to an instance of org.codehaus.groovy.grails.orm.hibernate.cfg.HibernateMappingBuilder, if you’re using Hibernate; otherwise, it’ll be an analogous NoSQL implementation if you’re using a different persistence provider, and that does have a version and a table method as expected. There’s no password method though. But there’s missing-method handling that looks for a field of the same name as the missing method, and when it finds a match and the parameter is a map, it uses the map data to configure the corresponding column.

So this lets us use an intuitive syntax composed of regular method calls that are handled by a delegate, usually doing a lot of work behind the scenes with a small amount of actual code.

Groovy’s Contributions in the War Against Verbosity

One of Groovy’s most popular features is its reduced verbosity compared to Java. It’s been said that Groovy is a “low ceremony” language. If you’re new to Groovy, you may not yet appreciate how much less code it takes to get things done compared to Java, especially if you use your IDE’s code-generation functions. But, once you get used to Groovy, you might find that you don’t even need to use an IDE anymore, except perhaps when you want to attach a debugger to work on a particularly gnarly bug.

You’ve already seen how property access and the compact syntax for collections and maps can help condense your code, but there are a lot more ways.


It’s rare to see constructors in Groovy classes. This is because the Groovy compiler adds a constructor that takes a Map and sets field values to map values where the key corresponds to a field name. This gives you named parameters for this constructor syntax, so it’s both more convenient and clearer which values are which. For example, a simple POGO like this:

class Person {
   String firstName
   String initial
   String lastName
   Integer age

can be constructed by setting some or all of the field values:

def author = new Person(firstName: 'Hunter', initial: 's', lastName: 'Thompson')
def illustrator = new Person(firstName: 'Ralph', lastName: 'Steadman', age: 76)
def someoneElse = new Person()

In the examples, I’m taking advantage of Groovy letting me omit the [ and ] map characters, because it makes the invocations cleaner.

This is especially useful for classes with many fields; in Java, you have to either define multiple constructors with various signatures or pass lots of nulls where you don’t have a value.

However, note that the Map constructor relies on the default constructor that’s added to all classes that don’t define any explicit constructors. It calls that constructor, then sets properties from the provided Map (this is defined in MetaClassImpl.invokeConstructor(), if you’re curious). But if you declare one or more parameterized constructors, the compiler doesn’t generate an empty one for you, and the Map constructor will fail.

Also, because it’s not a real constructor that’s added to the bytecode, you can use this with Java classes that have a default constructor, too. So you could replace this code:

MutablePropertyValues propertyValues = ...
def beanDef = new GenericBeanDefinition()

with this:

MutablePropertyValues propertyValues = ...
def beanDef = new GenericBeanDefinition(
      beanClassName: '',
      autowireMode: AbstractBeanDefinition.AUTOWIRE_BY_TYPE,
      propertyValues: propertyValues)

Checked Exceptions

Groovy also relaxes the requirement to catch and declare checked exceptions. Checked exceptions are widely regarded as a failure in the Java language, and a lot of the time, there isn’t much you can do once you catch one. So in Groovy, you don’t have to wrap calls to methods that throw checked exceptions in try/catch blocks, and you don’t have to declare checked exceptions in method signatures.

For example, consider java.sql.SQLException. A lot of the time, a SQLException will be caused by one of two things: temporary connectivity issues with the database and errors in your SQL. If you can’t connect to the database, you probably just have to punt and show an error page, and bad SQL is usually a development-time problem that you’ll fix. But you’re forced to wrap all JDBC code in try/catch blocks, thereby polluting your code.

You can still catch checked (and unchecked) exceptions in Groovy, and when you can handle an exception and retry or perform some action after catching it, you certainly should. It’s a good idea to also declare thrown exceptions in your method signatures, both for use by Java and also as a self-documenting code technique.

Groovy Truth

In Java, only boolean variables and expressions (including unboxed Boolean variables) can evaluate to true or false, for example, with if checks or as the argument to assert. But Groovy extends this in convenient ways. null object references evaluate to false. Nonempty collections, arrays, and maps; iterators and enumerations with more elements; matching regex patterns; Strings and GStrings (and other implementations of CharSequence) with nonzero length; and nonzero numbers and Characters will all evaluate to true.

This is especially helpful with strings as well as with collections and maps. So, for example, you can replace:

def someCollection = someMethod(...)
if (someCollection != null && !someCollection.isEmpty()) { ... }


def someCollection = someMethod(...)
if (someCollection) { ... }


String s = someMethod(...)
if (s != null && s.length() > 0) { ... }


String s = someMethod(...)
if (s) { ... }


Semicolons are for the most part unnecessary in Groovy, the exception being the traditional for loop (although you’ll most likely prefer the semicolon-free Groovy for/in version). Also, if you want to have multiple statements on one line, you still need to delimit them with semicolons.

Optional Return

You can omit the return keyword in a method or closure, because Groovy treats the last expression value as the return value if you don’t use return.


Scope modifiers are often omitted in Groovy because the default scope is public. You can still define private or protected fields and methods. Because package scope is the default in Java and there’s no keyword for that, Groovy added the groovy.transform.PackageScope annotation in version 1.8 for classes, methods, and fields.


You can often omit parentheses in method calls. This is only true if the method has arguments, because otherwise the call would look like property access. So, for example, all but the last of these are valid:

println("Using parentheses because I can")
println "Omitting parentheses because I can"
println // not valid; looks like property access
        // for a nonexistent getPrintln() method

You can’t omit parentheses from the right side of an assignment, however:

int sum = MathUtils.add(2, 2) // ok
int product = MathUtils.multiply 2, 2 // invalid, doesn't compile

Default Imports

Another space saver is the extended list of default imports. Java automatically imports everything from the java.lang package, and Groovy extends this to include*,*, java.util.*, groovy.lang.*, and groovy.util.*, as well as the java.math.BigDecimal and java.math.BigInteger classes.

Differences Between Java and Groovy

In general, you can rename a .java source file to .groovy and it will still be valid, although there are a few exceptions.

Array Initialization

Because Groovy uses braces to declare closures, you cannot initialize an array the standard Java way:

int[] oddUnderTen = { 1, 3, 5, 7, 9 };

Instead, we create the array using List syntax and cast it to the correct array type:

int[] oddUnderTen = [1, 3, 5, 7, 9]


def oddUnderTen = [1, 3, 5, 7, 9] as int[]

in and def Keywords

One other gotcha is that in is a keyword, used by the Groovy for loop, (e.g., for (bar in bars) { ... }); def is also a keyword. So Java code that uses either of these as a variable name will need to be updated.

do/while Loops

There is also no do/while loop in Groovy, so any code like this will need to be reworked:

do {
   // stuff
while (<truth expression>);

for Loops

Another small difference is that you can’t initialize more than one variable in the first part of a for loop, so this is invalid:

for (int count = someCalculation(), i = 0; i < count; i++) {

and you’ll need to initialize the count variable outside the loop (a rare case where Groovy is more verbose than Java!):

int count = someCalculation()
for (int i = 0; i < count; i++) {

or you could just skip the whole for loop and use times:

someCalculation().times {

or a range with a loop, if you need access to the loop variable:

for (i in 0..someCalculation()-1) {


Annotation values that have array types use a different syntax in Groovy than Java. In Java, you use { and } to define multivalued attributes:


but because these are used to define closures in Groovy, you must use [ and ] instead:


Groovy Equality

The previous examples will cause compilation errors if you rename a .java class to .groovy and try to compile it, or copy/paste Java code into an existing Groovy class. But checking for object equality actually works differently in Groovy.

In Java, == is mostly used for comparing numbers and other primitives, because comparing objects with == just compares object references but not the data in the instances. We use the equals method to test if two objects are equivalent and can be considered equal even though they’re not the same instances. But it’s rare to need the == object comparison, so Groovy overloads it to call equals (or compareTo, if the objects implement Comparable). And, if you do need to check that two references are the same object, use the is method—e.g.,

Groovy’s == overload is convenient and avoids having to check for null values, but because it works differently than the Java operator, you might want to consider not using it. It’s simple enough to replace x == y with x?.equals(y), which isn’t that many more characters and is still null-safe. Working with both Java and Groovy will keep you from introducing subtle bugs in your Java code. (I’m speaking from experience here….)

Multimethod Dispatch

Overloaded method selection is another runtime difference between Java and Groovy. Java’s type checking is stricter, so it uses the compilation type of a variable to choose which method to call, whereas Groovy uses the runtime type, because it’s dynamically checking all method invocations for metamethods, invokeMethod interception, etc.

So, for example, consider a few versions of a close utility method:

void close(Closeable c) {
   try { c.close() }
   catch (e) { println "Error closing Closeable" }

void close(Connection c) {
   try { c.close() }
   catch (e) { println "Error closing Connection" }

void close(Object o) {
   try { o.close() }
   catch (e) { println "Error closing Object" }

In Java, this code will invoke the close(Object) variant, because the compiler only knows the compile-time type of the connection variable:

Object connection = createConnection(); // a method that returns a Connection
// work with the connection

But, if this were Groovy, the close(Connection) method would be chosen, because it’s resolved at runtime and is based not on the compile type but the actual runtime type of the connection. This is arguably a better approach, but because it’s different from the Java behavior, it’s something that you should be aware of.

Groovy Strings

There are multiple ways to express string literals in Groovy. The approach used in Java—double-quoted strings—is supported of course, but Groovy also lets you use single quotes if you prefer. Multiline strings (sometimes called heredocs in other languages) are also supported, using triple quotes (either single or double). GStrings make things a lot more interesting, though.

The biggest benefit of GStrings is avoiding the clumsy string concatenation that’s required in Java:

String fullName = person.getFirstName() + " ";
if (person.getInitial() != null) {
   fullName += person.getInitial() + " ";
fullName += person.getLastName();

Using a StringBuilder (the preferred approach when concatenating in a loop) wouldn’t be much better in this case. But using a GString (along with property syntax), we can join the data in a single line of code:

def fullName = "$person.firstName ${person.initial ? person.initial + ' '
: ''}$person.lastName"

GStrings also work with multiline strings as long as you use three double quotes; this is convenient for tasks such as filling in templates for emails:

def template = """\
Dear $name,

Thanks for signing up for the Ralph's Bait and Tackle online store!
We appreciate your business and look forward to blah blah blah …


Here, I’m using the backslash character at the beginning of the string to avoid having an initial blank line. You can use three single quotes to create multiline strings, but they behave like regular strings that use single quotes, in that they do not support expression replacement.

Using the subscript operator lets you conveniently access substrings:

String str = 'Groovy Strings are groovy'
assert str[4] == 'v' // a String of length 1, not a char
assert str[0..5] == 'Groovy' // the first 6 chars
assert str[19..-1] == 'groovy' // the last 6 chars
assert str[15..17] == 'are' // a substring in the middle
assert str[17..15] == 'era' // a substring in the middle, reversed

Static this

Unlike Java where the this keyword only makes sense in instance scope, this resolves to the class in static scope. One use of this feature is when defining static loggers. In Log4j and SLF4J, you can define a logger with a class or the class name (or any string you like), but in Java, there’s no way (no convenient one anyway) to get the class name in static scope. This can lead to copy/paste problems. For example:

private static final Logger LOG = Logger.getLogger(Foo.class);

has the class hardcoded, so if you forget to change it and copy that to a different class, you’ll be logging as the wrong category. Instead, in Groovy, you can use:

private static final Logger LOG = Logger.getLogger(this)

which is more portable (and similar to the analogous instance version private final Logger log = Logger.getLogger(getClass())).

The Groovy JDK (GDK)

The Groovy JDK (GDK) is a Javadoc-style set of pages that describe methods added to the metaclass of JDK classes to extend them with extra functionality and make them easier to work with. There are currently over 1,000 methods listed. I strongly encourage you to check out the information there and familiarize yourself with what’s available. You may find that you’ve coded something that was already available and, in general, will probably realize that you’re working harder than you need to by not taking advantage of these built-in methods and features.

DefaultGroovyMethods and InvokerHelper

Many of the methods added to JDK metaclasses are implemented in the org.codehaus.groovy.runtime.DefaultGroovyMethods class. At its largest, this was a gigantic class (over 18,000 lines) with around 1,000 methods. In recent versions of Groovy, this large class is being refactored into several more focused classes, including ResourceGroovyMethods, IOGroovyMethods, StringGroovyMethods, and others. Many of the convenience methods that you use on a regular basis are implemented here, for example, the sort method that’s added to the Collection interface (it sorts lists and creates sorted lists from nonlist collections) is implemented by the public static <T> List<T> sort(Collection<T> self) method. It’s interesting to browse this class to see how things work under the covers, and you can use these methods yourself (although this is an internal class, so there may be some risk using it directly, because it’s not a public API class).

org.codehaus.groovy.runtime.InvokerHelper is another utility class with a lot of interesting functionality that you should check out.

Metaprogramming and the MOP

Groovy’s Meta Object Protocol (MOP) is the key to Groovy’s power and it’s what enables most of its coolest features. Every class gets a metaclass, which intercepts all method calls and enables customization of how methods are invoked, and also enables adding or removing methods. This is what makes Groovy a dynamic language; unlike Java, which compiles methods into class bytecode and doesn’t allow changes at runtime. Because Groovy’s MOP is intercepting all method calls, it can simulate adding a method as if it had been compiled in at startup. This makes JVM classes open classes that can be modified at any time.

And that’s just runtime metaprogramming; with compile-time metaprogramming using Abstract Syntax Tree (AST) transformations, you can also add actual methods to the class bytecode that are visible from Java.

Every Groovy class implements the groovy.lang.GroovyObject interface (it’s added by the compiler) that includes these methods:

Object invokeMethod(String name, Object args)
Object getProperty(String property)
void setProperty(String property, Object newValue)
MetaClass getMetaClass()
void setMetaClass(MetaClass metaClass)


Java classes can also implement GroovyObject to add Groovy-like behavior. The most convenient approach for this is to subclass the groovy.lang.GroovyObjectSupport adapter class, which implements the interface and provides sensible default implementations of the methods that can be overridden as needed.

When you invoke a method in Groovy (including accessing a property, because that calls the corresponding getter method), it’s actually dispatched to the object’s metaclass. This provides an AOP-like interception layer. The calls are implemented with reflection, which is slower than direct method invocation. But each new release of Java adds reflection speed improvements, and Groovy has several optimizations to reduce the cost of this overhead, the most significant being call site caching. Early versions of Groovy were quite slow, but modern Groovy has seen huge performance boosts and is often nearly as fast as Java. And because network latency and database access tend to contribute most to total web request time, the small increase in invocation time that Groovy can add tends to be insignificant, because it’s such a small percentage of the total time.

Adding Methods

The syntax for adding a method at runtime is essentially just one that registers a closure in the metaclass that’s associated with the specified method name and signature:

List.metaClass.removeRight = { int index ->
   delegate.remove(delegate.size() - 1 - index)

The List interface has a remove method, but this addition removes the item considering the position from the right instead of the left like remove:

assert 3 == [1, 2, 3].removeRight(0)
assert 2 == [1, 2, 3].removeRight(1)
assert 1 == [1, 2, 3].removeRight(2)

Recall that closures have a delegate that handles method calls invoked inside the closure. When adding methods to the metaclass, you can access the instance in which closure is invoked with the delegate property; in this example, it’s the list instance that removeRight is called on.

Intercepting Method Calls

Because all Groovy objects implement GroovyObject, you can override the invokeMethod method in your class to handle method invocations. There are a few variants of behavior though. By default, it’s only called for methods that don’t exist (analogous to methodMissing, which we’ll see in a bit), so for example:

class MathUtils {

   int add(int i, int j) { i + j }

   def invokeMethod(String name, args) {
      println "You called $name with args $args"

def mu = new MathUtils()
println mu.add(2, 3)
println mu.multiply(2, 3)

will generate this output:

You called multiply with args [2, 3]

because there is an add method but no multiply. If we change the class to implement the GroovyInterceptable marker interface (which extends GroovyObject):

class MathUtils implements GroovyInterceptable {

then the result is a java.lang.StackOverflowError. Hmmm. What’s up there? We tend to think of println as just an alias for System.out.println, but it’s actually a metamethod added to the Object class that calls System.out.println, so it will be intercepted along with the calls to add and multiply. So the fix is to use System.out.println directly:

class MathUtils implements GroovyInterceptable {

   int add(int i, int j) { i + j }

   def invokeMethod(String name, args) {
      System.out.println "You called $name with args $args"

def mu = new MathUtils()
println mu.add(2, 3)
println mu.multiply(2, 3)

and then we’ll see this output:

You called add with args [2, 3]
You called multiply with args [2, 3]

getProperty and setProperty

Overriding getProperty and/or setProperty always intercepts the property gets and sets, so the output of:

class Person {
   private String name

   def getProperty(String propName) {
      println "getProperty $propName"
      if ('name'.equals(propName)) {

   void setProperty(String propName, value) {
      println "setProperty $propName -> $value"
      if ('name'.equals(propName)) { = value

def p = new Person(name: 'me')

will be:

getProperty name

You might have expected to see output indicating that setProperty was called, since the map constructor is used and it sets property values from the map, in this case the name property to 'me'. But the implementation of this feature bypasses setProperty (this seems like a bug). But if you explicitly set the property: = 'you'

it works as expected:

setProperty name -> you

methodMissing and propertyMissing

GroovyObject doesn’t have methodMissing or propertyMissing methods, but if you implement one or both of them, they’ll be called for undefined method calls and property accesses. The signatures are similar to invokeMethod and getProperty:

class Person {
   String name

   def propertyMissing(String propName) {
      if ('eman'.equals(propName)) {
         return name.reverse()
      throw new MissingPropertyException(propName, getClass())

   def methodMissing(String methodName, args) {
      if ('knight'.equals(methodName)) {
         name = 'Sir ' + name
      throw new MissingMethodException(methodName, getClass(), args)

def p = new Person(name: 'Ralph')
println p.eman


which results in the output:

Sir Ralph

and, if you try to access a property or method that doesn’t exist or have special handling (e.g., println p.firstName or p.king()), then you’ll get the standard MissingPropertyException or MissingMethodException.

There are also static versions of methodMissing and propertyMissing, $static_methodMissing and $static_propertyMissing.

class Person {
   String name

   static $static_propertyMissing(String propName) {
      println "static_propertyMissing $propName"

   static $static_methodMissing(String methodName, args) {
      println "static_methodMissing $methodName"


The output from the above code is:

static_propertyMissing foo
static_methodMissing foo
static_propertyMissing bar

$static_methodMissing works slightly differently from methodMissing in that if there’s no method with the specified name, it looks for a closure property with that name to invoke as if it were a method. This results in a message about a missing foo property and a missing foo method.


Groovy adds several operators to the standard set of Java operators.

Null-Safe Dereference

The most commonly used is the null-safe dereference operator, ?., which lets you avoid a NullPointerException when calling a method or accessing a property on a null object. It’s especially useful in a chain of such accesses where a null value could occur at some point in the chain.

For example, you can safely call:

String name = person?.organization?.parent?.name

and if person, person.organization, or organization.parent are null, then null is returned as the expression value. The Java alternative is a lot more verbose:

String name = null;
if (person != null) {
   if (person.getOrganization() != null) {
      if (person.getOrganization().getParent() != null) {
         name = person.getOrganization().getParent().getName();


The Elvis operator, ?:, lets you condense ternary expressions; these two are equivalent:

String name = ?: defaultName


String name = ? : defaultName

They both assign the value of to the name variable if it is “Groovy true” (in this case, not null and has nonzero length, because it’s a string), but using the Elvis operator is more DRY.


The spread operator, *., is convenient when accessing a property or calling a method on a collection of items and collecting the results. It’s essentially a shortcut for the collect GDK method, although it’s limited to accessing one property or calling one method, for example:

def numbers = [1.41421356, 2.71828183, 3.14159265]
assert [1, 2, 3] == numbers*.intValue()


The spaceship operator <=> is useful when comparing values; for example, when implementing the compareTo method of the Comparable interface. For example, given a POGO where you want to sort by two properties, the spaceship operator makes the implementation very compact:

class Person implements Comparable<Person> {
   String firstName
   String lastName

   int compareTo(Person p) {
       lastName <=> p?.lastName ?: firstName <=> p?.firstName

   String toString() { "$firstName $lastName" }

def zakJones = new Person(firstName: 'Zak', lastName: 'Jones')
def jedSmith = new Person(firstName: 'Jed', lastName: 'Smith')
def alJones = new Person(firstName: 'Al', lastName: 'Jones')

def persons = [zakJones, jedSmith, alJones]
assert [alJones, zakJones, jedSmith] == persons.sort(false)

because the operator returns –1 if the left is less than the right, 0 if they’re equal, and 1 if the right is more than the left. So when the first expression (lastName <=> p?.lastName) is nonzero, its value is used as the return value and the sort is done by lastName. If the last names match, then the Elvis operator kicks in and the second expression (firstName <=> p?.firstName) is used to do a secondary sort by firstName.

You can also use the operator for one-off sorting regardless of whether the items are Comparable, for example:

assert [alJones, jedSmith, zakJones] == persons.sort(
    false, { a, b -> a?.firstName <=> b?.firstName })

which sorts just by firstName. Of course Groovy being Groovy, there’s a shorter way of doing that:

assert [alJones, jedSmith, zakJones] == persons.sort(false, { it.firstName })

Field Access

If you have a need to bypass a getter method (or if there is none) and directly access a field, you can use the .@ operator. For example, this class uses some simple logic to return a default value if none is specified, but if you want to know if the value is unspecified, you still can:

class Thing {

   private static final String DEF_NAME = 'foo'

   String name

   String getName() { name == null ? DEF_NAME : name }

assert 'bar' == new Thing(name: 'bar').name
assert 'foo' == new Thing().name
assert null == new Thing().@name

Note, however, that this operator only works on the current class; if the field is in a subclass, the operator cannot access it, and you have to use standard reflection.


The as operator is very useful, because it can perform many type coercions. For example, there’s no native syntax for a Set like there is for a List, but you can use as with List syntax to create a Set:

def things = ['a', 'b', 'b', 'c'] as Set
assert things.getClass().simpleName == 'HashSet'
assert things.size() == 3


The in operator is a convenient shortcut for the contains method in a collection:

assert 1 in [1, 2, 5]
assert !(3 in [1, 2, 5])

Method Reference

The .& operator lets you get a reference to a method and treat it like a closure. This might be useful if you’re working with higher order methods where you pass a closure as a parameter and want the option to pass a method; the .& operator creates an instance of org.codehaus.groovy.runtime.MethodClosure that invokes your method when it’s invoked.

class MathUtils {
   def add(x, y) { x + y }

def doMath(x, y, Closure c) {
   c(x, y)

def add = new MathUtils().&add
def multiply = { x, y -> x * y }

assert 8 == doMath(4, 2, multiply)
assert 6 == doMath(4, 2, add)
assert 2 == doMath(4, 2, { x, y -> x / y })

Overload Your Operators

Operator overloading is a powerful technique for compressing code, ideally in an intuitive way. It’s important that if you add an operator overload that it make sense—be sure to think about how cryptic the code can get if you add an operator overload that isn’t an appropriate choice.

The general approach for creating an operator overload is to implement the method that corresponds to the operator. The method must return this (or another instance) to work correctly. So, for example, if we have a Person class that has a collection of children:

class Person {
   String name
   List children = []

Adding a child to a Person instance is simple enough:

def parent = new Person(...)
def child = new Person(...)
parent.children.add child // or parent.children << child

But we can use the left-shift operator here to add a child:

class Person {
   String name
   List children = []

   def leftShift(Person child) {
      children << child

and then the code becomes simply:

def parent = new Person(...)
def child = new Person(...)
parent << child

The plus method would be another possibility (or you might implement both).

class Person {
   String name
   List children = []

   def plus(Person child) {
      children << child

and then you would use it like this:

def parent = new Person(...)
def child = new Person(...)
parent += child

because parent += child is the equivalent of parent = parent + child. Note that internally we’re still using << to add the child to the children list instead of switching to +=, because += creates a new List and copies the old list into it and then adds the new one. This is a lot more expensive than just adding to the current instance and should be avoided in general unless you have a reason to create a new list instance.

Table 1-1 shows the available overloadable operators and their corresponding implementation methods.

Table 1-1. Overloadable operators
OperatorImplementation method

a + b

a - b


a * b


a ** b


a / b


a % b


a | b


a & b


a ^ b


a++ or ++a

a-- or --a




a[b] = c

a.putAt(b, c)

a << b


a >> b


switch(a) { case(b) : }








Being Too Groovy

There’s a natural tendency to embrace Groovy fully once it becomes apparent how much it has to offer over Java. Writing highly idiomatic Groovy code can lead to the code being hard to understand, though. I’ve written cryptic code with no comments that I’ve looked at months later and had to relearn how it works as if someone else had written it, because it was old enough that I didn’t remember working on it, and I had sabotaged myself by writing the code in a way that made sense at the time but not when I came back to it.

The phrase I use for this is “be lazy but not sloppy.” By this, I mean save yourself time (and typing) and take advantage of Groovy’s cool features—but don’t overdo it and make your code hard to work with and understand.

def Considered Harmful

One example of being “too groovy” is overusing the def keyword. Optional typing is very convenient, but specifying the type can help other readers of your code (and even yourself). Naming variables and methods well makes code more self-documenting, and the same goes for whether to type variables. For example, consider this relatively information-free line of code:

def foo = bar(5, true)

It’s not at all clear what foo is or what you can do with it. If it’s a string, call it a String (or whatever the type is).

I usually don’t type both sides of an assignment, so because it’s clear that strings is a List from the right side of the assignment, I’m okay with:

def strings = []

but when the right side is a method invocation, I’ll type the left:

List<String> strings = someMethod('Hello', 'Groovy', 'World')

and I often add the generic type even though Groovy ignores it—again as a self-documentation practice and not because it has any other runtime effect. The same goes for the return type and parameter type(s) of methods; if it’s void, I specify void someMethod(...) instead of def someMethod(...), so the caller knows that there’s nothing being returned.

each is a convenient way of looping, but I rarely use it, because it has almost no benefit over the for/in loop. For example, I would use:

for (string in strings) { ... }

instead of:

strings.each { string -> ... }

because they’re equivalent, basically the same number of characters, and both are null-safe. And the for loop has the benefit that you can break out of it if there’s a reason to stop looping, whereas each cannot, because returning from the closure that you pass to the each method returns from the closure, not each.

Of course, these are arguments about preferences—there’s no right or wrong here. And I will certainly drop the type of a method parameter if it makes testing easier by letting me substitute a more convenient value that uses duck typing.

Closures Versus Methods

Another example of being “too groovy” is using a closure as a method where you don’t use any features of the closure. If you don’t set the delegate or use any other closure-specific feature, then there’s no reason to use:

def foo = { <params> ->

instead of:

<return type> foo(<params>) {

and, in fact, using the method has the not-insignificant benefit of letting you specify the return type. Plus, things like AOP and method proxying that aren’t Groovy-aware won’t work at all with closures, because they’re only treated like methods by Groovy—they’re just public fields and are ignored by Java.

One real example of this is Grails services. Unlike controllers and taglibs, services are implemented with methods. A transactional service is implemented by Spring as a proxy that subclasses your service class and intercepts all method calls to start or join a transaction as needed and manage error handling, automatic rollbacks, and so on. If you have a public closure in a service, it will be callable from Groovy just like a method, but it will not be transactional. The proxy only works with methods and completely ignores the closures, so you will introduce bugs that can be hard to track down by using closures here.

TypeChecked, CompileStatic, and invokedynamic

Groovy 2.0 and 2.1 add new features that make your code faster and provide more compiler checks. Groovy is a dynamic language and, as we’ve seen, this adds a tremendous amount of power and flexibility. But there are costs to this flexibility. One is that it’s easier to introduce typos and mistakes into Groovy code than Java, because the compiler is more forgiving. For example, a one-character mistake such as:

int hc = someObject.hashcode()

will compile but fail with a groovy.lang.MissingMethodException at runtime (unless there is actually a hashcode method in the class). The compiler doesn’t catch the mistake, because the code satisfies the Groovy grammar, but the compiler cannot know whether a hashcode method will be added to the metaclass before its first use in application code. And it can’t assume that you meant to call the hashCode method.

Good testing should find errors like this, but Groovy now provides an option to make the compiler more aggressive: the @TypeChecked annotation. This can be applied at the class level or on individual methods, and the code within the scope of the annotation will be compiled more like Java than Groovy. You lose flexibility with this annotation but add earlier error checking.

@CompileStatic is the other new interesting annotation in 2.0. This adds the same checks as the @TypeChecked annotation and also compiles your Groovy code to nearly the same bytecode as that from the equivalent Java code. This means that you lose Groovy’s metaprogramming support and some other dynamic features (although you retain many of the syntactic sugar features such as list and map comprehensions) but will see Java-like performance for the annotated code. Code that you would previously write in Java for maximum performance can now be written in Groovy.

Groovy 2.0 and 2.1 also have support for the new invokedynamic bytecode instruction that was added to support dynamic languages like Groovy and improve performance automatically. This differs from using @CompileStatic in that you don’t make any changes to your code. Instead, you use a different compiler and runtime JAR. The “indy” version of Groovy takes advantage of the existence of the invokedynamic instruction in JDK 7 and later (with performance being much better in JDK 8).

See the Groovy 2.0 release notes and Groovy 2.1 release notes for more information about these and other new features.

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