Java Extreme Programming Cookbook

Testing Asynchronous Methods

Problem

You want to test asynchronous methods.

Solution

Use a mock listener to wait for the asynchronous method to complete.

Discussion

An asynchronous method executes in its own thread, notifying some listener when it is complete. Code that calls an asynchronous method does not block, meaning that you cannot write a test like this:

public void testSomething(  ) {
    someAsynchronousMethod(  );
    assertXXX(...);
}

The problem with this code lies in the fact that the assertXXX( ) is almost certainly executed before the thread started by someAsynchronousMethod( ) has a chance to do its work. We really need to do something like this:

1. Call an asynchronous method.

2. Wait until the method is complete.

3. Get the results.

• If the method times out, fail.

• Otherwise, check the results.

To illustrate, let’s look at a simple interface for searching. We assume that searching occurs in its own thread, notifying a SearchModelListener whenever the search is complete. Example 4-8 shows the API.

Example 4-8. SearchModel interface

public interface SearchModel {
    void search(Object searchCriteria, SearchModelListener listener);
}

The search( ) method is asynchronous, notifying the SearchModelListener when it is complete. Example 4-9 shows the code for the SearchModelListener interface.

Example 4-9. SearchModelListener interface

public interface SearchModelListener extends EventListener {
    void searchFinished(SearchModelEvent evt);
}

In order to test the search model, we must write a mock listener that ...