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2009-10-01T02:35:53.133
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2009-10-01T02:35:53.133
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@Kaddy, you have two problems with the code you posted: <ol> <li>You have multiple instances of UI, and</li> <li>Your locking is per-method, but it appears you want to lock across methods.</li> </ol> Let me explain both. First, a really quick explanation of <code>synchronized</code>. Consider this code: <pre><code>class MyClass { synchronized void myMethod() { // do something } } </code></pre> This is essentially shorthand for: <pre><code>class MyClass { void myMethod() { synchronized (this) { // do something } } } </code></pre> How <code>synchronized</code> works is that if multiple threads try to sync on the same instance, then only one is allow inside the sync block at a time. When thread that did go inside leaves the block -- either through regular execution or because of exception -- then one of the remaining threads is allowed to enter its sync block. This goes on until no threads are trying to acquire a lock on that instance. So to your first problem: each thread has its own instance of Ui. So the threads are sync'ing on different instances. You can fix that problem in two ways. Either share one instance of Ui: <pre><code>UI ui = new UI(); RunnableUI rm1 = new RunnableUI(ui); RunnableUI rm2 = new RunnableUI(ui); </code></pre> Or lock on something other than the UI instance: <pre><code>class UI { private static final Object LOCK = new Object(); public Object getLock() { return LOCK; } public void update() { synchronized (getLock()) { // do something } } ... } </code></pre> To your second problem. You are not locking broadly enough. If you opt for a shared UI instance, try this: <pre><code>class RunnableUI implements Runnable { private final UI _ui; RunnableUI(UI ui) { _ui = ui; } public void run() { synchronized (_ui) { _ui.update(20); _ui.stop(); } } } </code></pre> If you opt for a shared lock, try this: <pre><code>class RunnableUI implements Runnable { public void run() { UI ui = new UI(); synchronized (ui.getLock()) { ui.update(20); ui.stop(); } } } </code></pre>
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