synchronized原理

每个JAVA对象都有一把锁， 当有多个线程同时访问共享资源的时候， 需要synchronized 来控制安全性， synchronized 分 synchronized 方法 和synchronized 快，使用synchronized 块时， 一定要显示的获得该对象的锁（如synchronized （object))而方法则不需要。

　　JAVA 的内存模型是对每一个进程有一个主内存， 每个线程有自己的内存， 他们从主内存中取数据， 然后计算， 再存入主内存中。

　　并发问题如下：如果多个线程同事操作同一数据， A线程从主内存中取的I的值为1， 然后进行加1操作， 这时B线程也取I的值， 进行加2操作， 然后A存入2到主内存中， B也存入， 这样就覆盖了A的值（同数据库中的并发问题一样）。

　　解决办法是用synchronized ， 如用synchronized（I）。被synchronized 修饰的方法（块）把以下三步操作当成一个原子操作：取数据， 操作数据， 存数据。 我们知道原子操作是不可以被打断的， 所以其保证了数据一致性， 这样同一时间只有一个线程再执行， 对性能有一定的影响。这也是synchronized 的第二个作用：保证同一时间只有一个线程在运行。 当实现SOCKET连接的时候经常用到.

　 　JAVA中规定对非FLOAT, LONG的原始类型的取和存操作为原子操作。 其实就是对一个字（32位）的取，存位原始操作， 因为FLOAT, LONG为两个字节的长度， 所以其取， 存为非原子操作。 如果想把他们也变为原子操作， 可以用VOLATILE关键字来修饰.

Intrinsic Locks and Synchronization

Synchronization is built around an internal entity known as the intrinsic lock or monitor lock . (The API specification often refers to this entity simply as a "monitor.") Intrinsic locks play a role in both aspects of synchronization: enforcing exclusive access to an object's state and establishing happens-before relationships that are essential to visibility.

Every object has an intrinsic lock associated with it. By convention, a thread that needs exclusive and consistent access to an object's fields has to acquire the object's intrinsic lock before accessing them, and then release the intrinsic lock when it's done with them. A thread is said to own the intrinsic lock between the time it has acquired the lock and released the lock. As long as a thread owns an intrinsic lock, no other thread can acquire the same lock. The other thread will block when it attempts to acquire the lock.

When a thread releases an intrinsic lock, a happens-before relationship is established between that action and any subsequent acquistion of the same lock.

Locks In Synchronized Methods

When a thread invokes a synchronized method, it automatically acquires the intrinsic lock for that method's object and releases it when the method returns. The lock release occurs even if the return was caused by an uncaught exception.

You might wonder what happens when a static synchronized method is invoked, since a static method is associated with a class, not an object. In this case, the thread acquires the intrinsic lock for the Class object associated with the class. Thus access to class's static fields is controlled by a lock that's distinct from the lock for any instance of the class.

Synchronized Statements

Another way to create synchronized code is with synchronized statements . Unlike synchronized methods, synchronized statements must specify the object that provides the intrinsic lock:

public void addName(String name) {
    synchronized(this) {
        lastName = name;
        nameCount++;
    }
    nameList.add(name);
}

In this example, the addName method needs to synchronize changes to lastName and nameCount , but also needs to avoid synchronizing invocations of other objects' methods. (Invoking other objects' methods from synchronized code can create problems that are described in the section on Liveness .) Without synchronized statements, there would have to be a separate, unsynchronized method for the sole purpose of invoking nameList.add .

Synchronized statements are also useful for improving concurrency with fine-grained synchronization. Suppose, for example, class MsLunch has two instance fields, c1 and c2 , that are never used together. All updates of these fields must be synchronized, but there's no reason to prevent an update of c1 from being interleaved with an update of c2 — and doing so reduces concurrency by creating unnecessary blocking. Instead of using synchronized methods or otherwise using the lock associated with this , we create two objects solely to provide locks.

public class MsLunch {
    private long c1 = 0;
    private long c2 = 0;
    private Object lock1 = new Object();
    private Object lock2 = new Object();

    public void inc1() {
        synchronized(lock1) {
            c1++;
        }
    }

    public void inc2() {
        synchronized(lock2) {
            c2++;
        }
    }
}

Use this idiom with extreme care. You must be absolutely sure that it really is safe to interleave access of the affected fields.

Reentrant Synchronization

Recall that a thread cannot acquire a lock owned by another thread. But a thread can acquire a lock that it already owns. Allowing a thread to acquire the same lock more than once enables reentrant synchronization . This describes a situation where synchronized code, directly or indirectly, invokes a method that also contains synchronized code, and both sets of code use the same lock. Without reentrant synchronization, synchronized code would have to take many additional precautions to avoid having a thread cause itself to block.