Swing Hacks

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Chapter 4, File Choosers

#32 Preview ZIP and JAR Files

HACK

Before you read any further, I want to warn you that this is

one of the longest and most complicated hacks in the book. I

don’t want to scare you away, but the code is pretty dense.

Of course, you wouldn’t expect any less from a hacks book,

right? What you will learn from this hack, however, will let

you build custom filesystem views for any type of data-

source, including FTP, WebDAV, or even SQL databases. I

think the complexity is worth it. Plus, you’ll learn more about

how the

File object really works and how to hack it to pieces.

Build File Proxies

The JFileChooser uses a FileSystemView to access the real filesystem. This

view, unfortunately, assumes the existence of actual

java.io.File objects.

There is no way to represent a filesystem without

Files, which wouldn’t be a

problem except that

File is a real class with many methods, not a simple

interface. Fortunately,

File is not declared final. You can override each

method with your own version to redirect the calls to another object, and

that’s exactly how this hack works. By creating proxies around the real ZIP

file objects, you can fool the

FileSystemView into working with items that

aren’t real files.

ZIP files are represented in the

java.util.zip package by a ZipFile object

that contains one

ZipEntry for each compressed file it contains. To show the

compressed files in the chooser, each

ZipEntry is wrapped inside of a

ZipEntryFileProxy, which extends the real File class. It contains a reference

to the

ZipEntry, the enclosing ZipFile, a path string, and the File object of

the actual ZIP file, as shown in Example 4-1.

Example 4-1. A ZIP file proxy

import java.util.zip.*;

public class ZipEntryFileProxy extends File {

ZipFileProxy zip;

ZipFile zipfile;

String name, path;

File parent;

ZipEntry entry;

public ZipEntryFileProxy(ZipFileProxy zip, ZipFile zipfile,

String path, File parent) {

super("");

this.zip = zip;

this.zipfile = zipfile;

this.path = path;

this.parent = parent;

this.entry = zipfile.getEntry(path);

Preview ZIP and JAR Files #32

Chapter 4, File Choosers

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HACK

The constructor is pretty straightforward. It saves references to the passed-in

object and then does some calculations on the path. If the entry is a direc-

tory, then the path will end with a slash. The code chops off this trailing

slash, if present, and then looks for the name of the entry. Files inside of a

ZIP are stored as a list of complete pathnames with slashes, not as a tree of

objects like a real filesystem. Thus, the name of a file—what you would nor-

mally think of as its filename—is actually just the last part of a complete

path, including the parent directories. To parse this and get the real file-

name, the code looks for the last slash in the filename and pulls out the trail-

ing string.

The rest of the

ZipEntryFileProxy class overrides a portion of the standard

File methods. Each call returns a value based on the saved data from the

ZIP file, rather than the actual

File parent class:

public boolean exists( ) { return true; }

public int hashCode( ) {

return name.hashCode( ) ^ 1234321;

}

public String getName( ) { return name; }

public String getPath( ) { return path; }

public boolean isDirectory() { return entry.isDirectory( ); }

public boolean isAbsolute( ) { return true; }

public String getAbsolutePath( ) { return path; }

public File getAbsoluteFile( ) { return this; }

public File getCanonicalFile( ) { return this; }

public File getParentFile( ) { return parent; }

public boolean equals(Object obj) {

if(obj instanceof ZipEntryFileProxy) {

ZipEntryFileProxy zo = (ZipEntryFileProxy)obj;

// determine if the entry is a directory

String tmp = path;

if(entry.isDirectory( )) {

tmp = path.substring(0,path.length( )-1);

}

// then calculate the name

int brk = tmp.lastIndexOf("/");

name = path;

if(brk != -1) {

name = tmp.substring(brk+1);

}

}

Example 4-1. A ZIP file proxy (continued)