How to generate strings that share the same hashcode in Java?

Question

An existing system written in Java uses the hashcode of a string as its routing strategy for load balancing.

Now, I cannot modify the system but ne

Answer 1

You can instrument the java.lang.String class so its method hashCode() will always return the same number.

I suppose Javassist is the easiest way to do such an instrumentation.

In short:

• obtain an instance of java.lang.instrument.Instrumentation by using a Java-agent (see package java.lang.instrument documentation for details)
• redefine java.lang.String class by using Instrumentation.redefineClasses(ClassDefinition[]) method

The code will look like (roughly):

ClassPool classPool = new ClassPool(true);
CtClass stringClass = classPool.get("java.lang.String");
CtMethod hashCodeMethod = stringClass.getDeclaredMethod("hashCode", null);
hashCodeMethod.setBody("{return 0;}");
byte[] bytes = stringClass.toBytecode();
ClassDefinition[] classDefinitions = new ClassDefinition[] {new ClassDefinition(String.class, bytes);
instrumentation.redefineClasses(classDefinitions);// this instrumentation can be obtained via Java-agent

Also don't forget that agent manifest file must specify Can-Redefine-Classes: true to be able to use redefineClasses(ClassDefinition[]) method.