Intrinsic Locks and Synchronization
Synchronization is built around an internal entity known as the intrinsic（固有的，内在的，本质的） 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.
因为静态方法与类相关，与实例无关。因此，static synchronized method 将获得类的锁。
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.
相关单词：
interleave - 交错，交叉存取的，隔行扫描的
sole - 唯一
维基：双重检查锁定模式