Java线程 - 同步问题

Question

From Sun's tutorial:

Synchronized methods enable a simple strategy for preventing thread interference and memory consistency errors: if an object is visible to more than one thread, all reads or writes to that object's variables are done through synchronized methods. (An important exception: final fields, which cannot be modified after the object is constructed, can be safely read through non-synchronized methods, once the object is constructed) This strategy is effective, but can present problems with liveness, as we'll see later in this lesson.

Q1. Is the above statements mean that if an object of a class is going to be shared among multiple threads, then of that class (except getters of final fields) should be made synchronized, since instance methods process instance variables?

Answer 1

In order to understand concurrency in Java, I recommend the invaluable Java Concurrency in Practice .

In response to your specific question, although synchronizing all methods is a quick-and-dirty way to accomplish thread safety, it does not scale well at all. Consider the much maligned Vector class. Every method is synchronized, and it works terribly, because iteration is still not thread safe.

Answer 2

不是。这意味着同步方法是实现线程安全的一种方法，但它们不是唯一的方法，并且它们本身并不能保证在所有情况下都完全安全。

Answer 3

Not necessarily. You can synchronize (eg place a lock on dedicated object) part of the method where you access object's variables, for example. In other cases, you may delegate job to some inner object(s) which already handles synchronization issues.
There are lots of choices, it all depends on the algorithm you're implementing. Although, 'synchronized' keywords is usually the simplest one.

edit
There is no comprehensive tutorial on that, each situation is unique. Learning it is like learning a foreign language: never ends :)

But there are certainly helpful resources. In particular, there is a series of interesting articles on Heinz Kabutz's website.
http://www.javaspecialists.eu/archive/Issue152.html (see the full list on the page)

If other people have any links I'd be interested to see also. I find the whole topic to be quite confusing (and, probably, most difficult part of core java), especially since new concurrency mechanisms were introduced in java 5.

Have fun!

Answer 4

In the most general form yes.

Immutable objects need not be synchronized.

Also, you can use individual monitors/locks for the mutable instance variables (or groups there of) which will help with liveliness. As well as only synchronize the portions where data is changed, rather than the entire method.

Answer 5

synchronized methodName vs synchronized( object )

That's correct, and is one alternative. I think it would be more efficient to synchronize access to that object only instead synchronize all it's methods.

While the difference may be subtle, it would be useful if you use that same object in a single thread

ie ( using synchronized keyword on the method )

class SomeClass {
    private int clickCount  = 0;

    public synchronized void click(){
        clickCount++;
    }
 }

When a class is defined like this, only one thread at a time may invoke the click method.

What happens if this method is invoked too frequently in a single threaded app? You'll spend some extra time checking if that thread can get the object lock when it is not needed.

class Main {
    public static void main( String  [] args ) {
         SomeClass someObject = new SomeClass();
         for( int i = 0 ; i < Integer.MAX_VALUE ; i++ ) {
             someObject.click();
         }
    }
 }

In this case, the check to see if the thread can lock the object will be invoked unnecessarily Integer.MAX_VALUE ( 2 147 483 647 ) times.

So removing the synchronized keyword in this situation will run much faster.

So, how would you do that in a multithread application?

You just synchronize the object:

synchronized ( someObject ) {
    someObject.click();
}

Vector vs ArrayList

As an additional note, this usage ( syncrhonized methodName vs. syncrhonized( object ) ) is, by the way, one of the reasons why java.util.Vector is now replaced by java.util.ArrayList . Many of the Vector methods are synchronized.

Most of the times a list is used in a single threaded app or piece of code ( ie code inside jsp/servlets is executed in a single thread ), and the extra synchronization of Vector doesn't help to performance.

Same goes for Hashtable being replaced by HashMap

Answer 6

In fact getters a should be synchronized too or fields are to be made volatile . That is because when you get some value, you're probably interested in a most recent version of the value. You see, synchronized block semantics provides not only atomicity of execution (eg it guarantees that only one thread executes this block at one time), but also a visibility. It means that when thread enters synchronized block it invalidates its local cache and when it goes out it dumps any variables that have been modified back to main memory. volatile variables has the same visibility semantics.

Answer 7

No. Even getters have to be synchronized, except when they access only final fields. The reason is, that, for example, when accessing a long value, there is a tiny change that another thread currently writes it, and you read it while just the first 4 bytes have been written while the other 4 bytes remain the old value.

Answer 8

Yes, that's correct. All methods that modify data or access data that may be modified by a different thread need to be synchronized on the same monitor.

The easy way is to mark the methods as synchronized. If these are long-running methods, you may want to only synchronize that parts that the the reading/writing. In this case you would definie the monitor, along with wait() and notify().

Answer 9

The simple answer is yes. If an object of the class is going to be shared by multiple threads, you need to syncronize the getters and setters to prevent data inconsistency. If all the threads would have seperate copy of object, then there is no need to syncronize the methods. If your instance methods are more than mere set and get, you must analyze the threat of threads waiting for a long running getter/setter to finish.

Answer 10

You could use synchronized methods, synchronized blocks, concurrency tools such as Semaphore or if you really want to get down and dirty you could use Atomic References. Other options include declaring member variables as volatile and using classes like AtomicInteger instead of Integer .

It all depends on the situation, but there are a wide range of concurrency tools available - these are just some of them.

Synchronization can result in hold-wait deadlock where two threads each have the lock of an object, and are trying to acquire the lock of the other thread's object.

Synchronization must also be global for a class, and an easy mistake to make is to forget to synchronize a method. When a thread holds the lock for an object, other threads can still access non synchronized methods of that object.