Both methods lock the same monitor. Therefore, you can’t simultaneously execute them on the same object from different threads (one of the two methods will block until the other is finished).
I think its best to cite Java Concurrency in Practice here: It is a common mistake to assume that synchronization needs to be used only when writing to shared variables; this is simply not true. For each mutable state variable that may be accessed by more than one thread, all accesses to that variable must … Read more
The difference is simple: if the locked-on object is in a static field, then all instances of MyClass* will share that lock (i.e. no two objects will be able to lock on that object at the same time). If the field is non-static, then each instance will have its own lock, so only calls of … Read more
Yes, it is guaranteed that thread 2 will print “done” . Of course, that is if the write to b in Thread 1 actually happens before the read from b in Thread 2, rather than happening at the same time, or earlier! The heart of the reasoning here is the happens-before relationship. Multithreaded program executions … Read more
First of all, I encourage you to really try hard to deal with concurrency issues on a higher level of abstraction, i.e. solving it using classes from java.util.concurrent such as ExecutorServices, Callables, Futures etc. That being said, there’s nothing wrong with synchronizing on a non-final field per se. You just need to keep in mind … Read more
Servlet 2.5 spec: Multiple servlets executing request threads may have active access to the same session object at the same time. The container must ensure that manipulation of internal data structures representing the session attributes is performed in a threadsafe manner. The Developer has the responsibility for threadsafe access to the attribute objects themselves. This … Read more
If you declare the method as synchronized (as you’re doing by typing public synchronized void addA()) you synchronize on the whole object, so two thread accessing a different variable from this same object would block each other anyway. If you want to synchronize only on one variable at a time, so two threads won’t block … Read more
Just to add a little detail to Oscar’s (pleasingly succinct!) answer, the relevant section on the Java Language Specification is 8.4.3.6, ‘synchronized Methods’: A synchronized method acquires a monitor (ยง17.1) before it executes. For a class (static) method, the monitor associated with the Class object for the method’s class is used. For an instance method, … Read more
Without putting my brain fully into gear, from a quick scan of what you say it looks as though you need to intern() your Strings: final String firstkey = “Data-” + email; final String key = firstkey.intern(); Two Strings with the same value are otherwise not necessarily the same object. Note that this may introduce … Read more
Can anyone tell me the advantage of the synchronized method over the synchronized block with an example? Thanks. There is not a clear advantage of using synchronized method over the block. Perhaps the only one ( but I wouldn’t call it an advantage ) is you don’t need to include the object reference this. Method: … Read more