Loading a JAR dynamically from memory

Question

I want to load a JAR dynamically, straight for memory.
Say, I have a buffer that contains a JAR, and I want to load all the classes inside the JAR, or at least list all of the files that exist inside the JAR. (classes, images, etc...).
What do I do if the first class that I load depends on the second class, for example? Does java know how to handle this? Or I have take care of this by myself?

Answer 1

Since you said “at least list all of the files that exist inside the JAR”, let’s begin with that rather easy task.

Suppose, you have your JarFile in a byte array, byte[] buffer:

try(JarInputStream is=new JarInputStream(new ByteArrayInputStream(buffer))) {
    for(;;) {
        JarEntry nextEntry = is.getNextJarEntry();
        if(nextEntry==null) break;
        System.out.println(nextEntry);
    }
}

Loading classes from such a representation doesn’t work out-of-the-box because the standard ClassLoader implementations rely on the JarFile implementation which relies on a physical file rather than an abstraction.

So unless you simply write the buffer into a temporary file, it boils down to implement your own ClassLoader. Since the JRE supports only stream access as shown above, you will have to scan linearly to find a requested resource/class or iterate once and store the entries into a Map.

One alternative to implementing a ClassLoader is to implement a custom URL handler to use together with a URLClassLoader which reduces the task to the lookup as described above:

final Map<String,byte[]> map=new HashMap<>();
try(JarInputStream is=new JarInputStream(new ByteArrayInputStream(buffer))) {
    for(;;) {
        JarEntry nextEntry = is.getNextJarEntry();
        if(nextEntry==null) break;
        final int est=(int)nextEntry.getSize();
        byte[] data=new byte[est>0? est: 1024];
        int real=0;
        for(int r=is.read(data); r>0; r=is.read(data, real, data.length-real))
            if(data.length==(real+=r)) data=Arrays.copyOf(data, data.length*2);
        if(real!=data.length) data=Arrays.copyOf(data, real);
        map.put("/"+nextEntry.getName(), data);
    }
}
URL u=new URL("x-buffer", null, -1, "/", new URLStreamHandler() {
    protected URLConnection openConnection(URL u) throws IOException {
        final byte[] data = map.get(u.getFile());
        if(data==null) throw new FileNotFoundException(u.getFile());
        return new URLConnection(u) {
            public void connect() throws IOException {}
            @Override
            public InputStream getInputStream() throws IOException {
                return new ByteArrayInputStream(data);
            }
        };
    }
});
try(URLClassLoader cl=new URLClassLoader(new URL[]{u})) {
    cl.loadClass( « a class from your JarFile buffer »);
}