为什么这个同步块似乎需要很长时间才能锁定？

Question

I am new to multi-threading in java, and I have a question some might find trivial.

I have to debug a third party piece of code and I need some basic information, to know where to look for the problem because the code is very large.

When the following code runs:

public void method()
{
   long startTime = System.currentTimeMillis();
   synchronized (obj)
   {
      log( "time:" + System.currentTimeMillis() - startTime + " ms" );
      ...
   }
}

I get:

11:13:12 - time: 3816 ms
...
11:14:14 - time: 0 ms

Why is taking so long (3816 ms) to get the lock for the object? Where should I look? For example, I would imagine a possible answer would be to look for code which acquires the lock for "obj" ie block such as:

synchronized (obj) { ... }

Or is it possible that any modification at the object "obj" without "synchronized" can also lock the object?

Answer 1

If it takes a thread that long to obtain a lock, it's because someone else is currently holding it.

You should look for two things:

1. Code blocks which synchronize on the same object or on other references to it (known as synchronized statements ) :

    synchronized (obj) { ... } 

2. Synchronized methods within the object itself.

   Say obj is of type MyObject , then you should look for methods such as these:

    public class MyObject{ public synchronized void myMethod() { ... } } 

   Because they are essentially the same as

    public class MyObject{ public void myMethod() { synchronized (this) { ... } } } 

   So if a thread is executing obj.myMethod() , threads that want to enter a synchronized (obj) block will have to wait, as they all lock on the same object. This, by the way, is the reason why I strongly recommend never using the synchronized method syntax, and always locking over a private (or protected) class member.

If another thread is currently executing code within such a block, the current thread will be locked out until the other thread finishes.

You can use jvisualvm's Threads tab or Jstack to take snapshots of the current execution state of all threads and the locks they're holding. If you're on android, see this answer on how to get a thread dump on there.

Answer 2

The jstack utility that is part of the jdk can help with this. The -l (long listing) option will print all of the locks held by various threads. If you can catch your program in the midst of the issue then you can find the other thread that is holding the lock. You do this by finding your thread, seeing what condition object it is waiting on, then searching the rest of the stack traces for that condition object.

This article has more detailed information about how to look at the thread dump.

Answer 3

You need to check the following:

• Are there any method/block in your obj class, which is synchronized on this. If yes then there must be multiple threads where one is your above code, while other might be using the methods of same obj.
• Where all you are sharing obj? If it's shared by multiple classes, then check out who locks on same obj.