Threads may overwrite defineClassMethod in getDefineClassMethod method of AccessClassLoader

[detective0922]

	private static Method getDefineClassMethod() throws Exception {
		// DCL on volatile
		if (defineClassMethod==null) {
			synchronized(accessClassLoaders) {
				defineClassMethod = ClassLoader.class.getDeclaredMethod("defineClass", new Class[] {String.class, byte[].class, int.class,
				int.class, ProtectionDomain.class});
				try {
					defineClassMethod.setAccessible(true);
				}
				catch (Exception ignored) {
				}
			}
		}
		return defineClassMethod;
	}

When Thread 1 assign a value to defineClassMethod and leave the critical section, then Thread 2 may assign a new value to defineClassMethod.
Maybe should check defineClassMethod==null inside synchronized(accessClassLoaders) code block too to avoid this issue and complete the DCL.