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copied!<p>Java's intrinsic locks (used with the <code>synchronized</code> keyword) are re-entrant. The lock required to enter <code>r1</code> is the same lock as the one required to enter <code>r</code>. A thread which already owns a lock is allowed to re-acquire it. You can think of there being a counter for the number of times a thread has acquired the same lock. In method <code>r1</code>, the calling thread has acquired the same lock twice. The lock is only available to other threads once the "count" goes back to zero; that is, once the thread that obtained the lock has released it exactly as many times as it obtained it.</p> <p><strong>Edit</strong>: Here is an example of five threads that each bounce between two mutually-recursive methods that both require the same lock. By running this example you can see that no thread will enter <code>methodOne</code> until the thread before it has completely finished recursing, and that there is no problem for the same thread to continually enter the same (or different) <code>synchronized</code> methods that share the same lock.</p> <pre><code>public class LockExample { private static synchronized void methodOne(int depth) { if (depth == 0) { return; } else { System.out.println(Thread.currentThread().getName() + " methodOne (in), depth " + depth); methodTwo(depth - 1); System.out.println(Thread.currentThread().getName() + " methodOne (out), depth " + depth); } } private static synchronized void methodTwo(int depth) { if (depth == 0) { return; } else { System.out.println(Thread.currentThread().getName() + " methodTwo (in), depth " + depth); methodOne(depth - 1); System.out.println(Thread.currentThread().getName() + " methodTwo (out), depth " + depth); } } public static void main(String[] args) { for (int i = 0; i < 5; i++) { Thread t = new Thread(new Runnable() { @Override public void run() { methodOne(10); } }); t.setName("Thread" + i); t.start(); } } } </code></pre> <p>Sample output (sorry for length):</p> <pre><code>Thread0 methodOne (in), depth 10 Thread0 methodTwo (in), depth 9 Thread0 methodOne (in), depth 8 Thread0 methodTwo (in), depth 7 Thread0 methodOne (in), depth 6 Thread0 methodTwo (in), depth 5 Thread0 methodOne (in), depth 4 Thread0 methodTwo (in), depth 3 Thread0 methodOne (in), depth 2 Thread0 methodTwo (in), depth 1 Thread0 methodTwo (out), depth 1 Thread0 methodOne (out), depth 2 Thread0 methodTwo (out), depth 3 Thread0 methodOne (out), depth 4 Thread0 methodTwo (out), depth 5 Thread0 methodOne (out), depth 6 Thread0 methodTwo (out), depth 7 Thread0 methodOne (out), depth 8 Thread0 methodTwo (out), depth 9 Thread0 methodOne (out), depth 10 Thread2 methodOne (in), depth 10 Thread2 methodTwo (in), depth 9 Thread2 methodOne (in), depth 8 Thread2 methodTwo (in), depth 7 Thread2 methodOne (in), depth 6 Thread2 methodTwo (in), depth 5 Thread2 methodOne (in), depth 4 Thread2 methodTwo (in), depth 3 Thread2 methodOne (in), depth 2 Thread2 methodTwo (in), depth 1 Thread2 methodTwo (out), depth 1 Thread2 methodOne (out), depth 2 Thread2 methodTwo (out), depth 3 Thread2 methodOne (out), depth 4 Thread2 methodTwo (out), depth 5 Thread2 methodOne (out), depth 6 Thread2 methodTwo (out), depth 7 Thread2 methodOne (out), depth 8 Thread2 methodTwo (out), depth 9 Thread2 methodOne (out), depth 10 Thread3 methodOne (in), depth 10 Thread3 methodTwo (in), depth 9 Thread3 methodOne (in), depth 8 Thread3 methodTwo (in), depth 7 Thread3 methodOne (in), depth 6 Thread3 methodTwo (in), depth 5 Thread3 methodOne (in), depth 4 Thread3 methodTwo (in), depth 3 Thread3 methodOne (in), depth 2 Thread3 methodTwo (in), depth 1 Thread3 methodTwo (out), depth 1 Thread3 methodOne (out), depth 2 Thread3 methodTwo (out), depth 3 Thread3 methodOne (out), depth 4 Thread3 methodTwo (out), depth 5 Thread3 methodOne (out), depth 6 Thread3 methodTwo (out), depth 7 Thread3 methodOne (out), depth 8 Thread3 methodTwo (out), depth 9 Thread3 methodOne (out), depth 10 Thread1 methodOne (in), depth 10 Thread1 methodTwo (in), depth 9 Thread1 methodOne (in), depth 8 Thread1 methodTwo (in), depth 7 Thread1 methodOne (in), depth 6 Thread1 methodTwo (in), depth 5 Thread1 methodOne (in), depth 4 Thread1 methodTwo (in), depth 3 Thread1 methodOne (in), depth 2 Thread1 methodTwo (in), depth 1 Thread1 methodTwo (out), depth 1 Thread1 methodOne (out), depth 2 Thread1 methodTwo (out), depth 3 Thread1 methodOne (out), depth 4 Thread1 methodTwo (out), depth 5 Thread1 methodOne (out), depth 6 Thread1 methodTwo (out), depth 7 Thread1 methodOne (out), depth 8 Thread1 methodTwo (out), depth 9 Thread1 methodOne (out), depth 10 Thread4 methodOne (in), depth 10 Thread4 methodTwo (in), depth 9 Thread4 methodOne (in), depth 8 Thread4 methodTwo (in), depth 7 Thread4 methodOne (in), depth 6 Thread4 methodTwo (in), depth 5 Thread4 methodOne (in), depth 4 Thread4 methodTwo (in), depth 3 Thread4 methodOne (in), depth 2 Thread4 methodTwo (in), depth 1 Thread4 methodTwo (out), depth 1 Thread4 methodOne (out), depth 2 Thread4 methodTwo (out), depth 3 Thread4 methodOne (out), depth 4 Thread4 methodTwo (out), depth 5 Thread4 methodOne (out), depth 6 Thread4 methodTwo (out), depth 7 Thread4 methodOne (out), depth 8 Thread4 methodTwo (out), depth 9 Thread4 methodOne (out), depth 10 </code></pre>

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