Does Java guarantee that Object.getClass() == Object.getClass()?

Question

I really do mean identity-equality here.

For example, will the following always print true?

System.out.println(\"foo\".getClass() ==         


        

Answer 1

It is guaranteed per classloader, as stated in the JVM specification:

First, the Java Virtual Machine determines whether it has already recorded that L is an initiating loader of a class or interface denoted by N. If so, this creation attempt is invalid and loading throws a LinkageError.

That is, if a class loader (L) tries to bypass default Class instances caching, and make the JVM load the byte[] definition more than once for the same class name (N), a LinkageError will be thrown by the JVM.

For example, implement a class loader that calls defineClass(...) each time loadClass(...) is invoked (bypassing default caching):

public class ClassloaderTest {

    private static final byte[] CLASS_DEF = readClassBytes();

    private static byte[] readClassBytes() {
        try {
            InputStream is = ClassloaderTest.class.getResourceAsStream("ClassloaderTest.class");
            ByteArrayOutputStream buffer = new ByteArrayOutputStream();
            int nRead;
            byte[] data = new byte[16384];
            while ((nRead = is.read(data, 0, data.length)) != -1) {
                buffer.write(data, 0, nRead);
            }
            buffer.flush();
            return buffer.toByteArray();
        } catch (IOException ex) {
            throw new AssertionError();
        }
    }

    private static ClassLoader createNonCachingClassloader() {
        return new ClassLoader() {
            @Override
            public Class loadClass(String name) throws ClassNotFoundException {
                if (name.equals("classloader.ClassloaderTest")) {
                    return defineClass(name, CLASS_DEF, 0, CLASS_DEF.length);
                } else {
                    return getParent().loadClass(name);
                }
            }
        };
    }

    public static void main(String[] args) throws Exception {
        ClassLoader cl = createNonCachingClassloader();
        Class cl1 = cl.loadClass("classloader.ClassloaderTest");
        Class cl2 = cl.loadClass("classloader.ClassloaderTest");
        System.out.println(cl1==cl2);
    }
}

and this is what happens:

Exception in thread "main" java.lang.LinkageError: loader (instance of  classloader/ClassloaderTest$1): attempted  duplicate class definition for name: "classloader/ClassloaderTest"
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:642)
    at classloader.ClassloaderTest$1.loadClass(ClassloaderTest.java:53)
    at classloader.ClassloaderTest.main(ClassloaderTest.java:64)

Cheers