ThreadLocal example

Debugger.java

import java.util.ArrayList;
import java.util.List;

public class Debugger {
    public static void main(String[] args) {
        ThreadGroup cleanupThreads = new ThreadGroup("Cleanup group");
        Thread t1 = new Thread(cleanupThreads, new Printer());
        Thread t2 = new Thread(cleanupThreads, new Printer());
        t1.start();
        t2.start();
    }
    private static class DebugLocal extends ThreadLocal<List<String>> {
        @Override
        protected List<String> initialValue() {
            return new ArrayList<String>();
        }
        List<String> getList() {
            return super.get();
        }
    }
    private static final DebugLocal local = new DebugLocal();
    public void put(String message) {
        local.getList().add(message);
    }
    public List<String> getMessages() {
        return local.getList();
    }
}

class Printer implements Runnable {
    @Override
    public void run() {
        new Foo().foo();
        new Bar().bar();
        System.out.println(new Debugger().getMessages());
    }
}

class Foo {
    void foo() {
        Debugger d = new Debugger();
        d.put("Some message here by foo");
    }
}

class Bar {
    void bar() {
        Debugger d = new Debugger();
        d.put("Some message here by bar");
    }
}