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
The snippet synchronized(X.class) uses the class instance as a monitor. As there is only one class instance (the object representing the class metadata at runtime) one thread can be in this block. With synchronized(this) the block is guarded by the instance. For every instance only one thread may enter the block. synchronized(X.class) is used to … 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
It’s important to understand that there are two aspects to thread safety. execution control, and memory visibility The first has to do with controlling when code executes (including the order in which instructions are executed) and whether it can execute concurrently, and the second to do with when the effects in memory of what has … Read more