Synchronized 关键字的用法

第一条:当一个线程访问某对象的synchronized方法或者synchronized代码块时，其他线程对该对象的该synchronized方法或者synchronized代码块的访问将被阻塞。

public class MoreThread {public static void main(String[] args) {Piao piao = new Piao();Thread t1 = new Thread(piao);Thread t2 = new Thread(piao);Thread t3 = new Thread(piao);t1.start();t2.start();t3.start();}
}class Piao implements Runnable {@Overridepublic void run() {/*synchronized(this) {*/for (int i = 0; i < 10; i++) {try {Thread.sleep(100);System.out.println(Thread.currentThread().getName() + " 卖票：ticket " + i);} catch (InterruptedException e) {e.printStackTrace();}}/*}*/}
}

　　没有加上synchronized：

Thread-1 卖票：ticket 0
Thread-0 卖票：ticket 0
Thread-2 卖票：ticket 0
Thread-1 卖票：ticket 1
Thread-0 卖票：ticket 1
Thread-2 卖票：ticket 1
Thread-1 卖票：ticket 2
Thread-0 卖票：ticket 2
Thread-2 卖票：ticket 2
Thread-1 卖票：ticket 3
Thread-0 卖票：ticket 3
Thread-2 卖票：ticket 3
Thread-0 卖票：ticket 4
Thread-2 卖票：ticket 4
Thread-1 卖票：ticket 4
Thread-2 卖票：ticket 5
Thread-0 卖票：ticket 5
Thread-1 卖票：ticket 5
Thread-1 卖票：ticket 6
Thread-2 卖票：ticket 6
Thread-0 卖票：ticket 6
Thread-1 卖票：ticket 7
Thread-0 卖票：ticket 7
Thread-2 卖票：ticket 7
Thread-1 卖票：ticket 8
Thread-0 卖票：ticket 8
Thread-2 卖票：ticket 8
Thread-1 卖票：ticket 9
Thread-2 卖票：ticket 9
Thread-0 卖票：ticket 9

　　加上synchronized：这个代码块要等一个线程访问完成之后，另一个线程才能访问。

Thread-1 卖票：ticket 0
Thread-1 卖票：ticket 1
Thread-1 卖票：ticket 2
Thread-1 卖票：ticket 3
Thread-1 卖票：ticket 4
Thread-1 卖票：ticket 5
Thread-1 卖票：ticket 6
Thread-1 卖票：ticket 7
Thread-1 卖票：ticket 8
Thread-1 卖票：ticket 9
Thread-0 卖票：ticket 0
Thread-0 卖票：ticket 1
Thread-0 卖票：ticket 2
Thread-0 卖票：ticket 3
Thread-0 卖票：ticket 4
Thread-0 卖票：ticket 5
Thread-0 卖票：ticket 6
Thread-0 卖票：ticket 7
Thread-0 卖票：ticket 8
Thread-0 卖票：ticket 9
Thread-2 卖票：ticket 0
Thread-2 卖票：ticket 1
Thread-2 卖票：ticket 2
Thread-2 卖票：ticket 3
Thread-2 卖票：ticket 4
Thread-2 卖票：ticket 5
Thread-2 卖票：ticket 6
Thread-2 卖票：ticket 7
Thread-2 卖票：ticket 8
Thread-2 卖票：ticket 9

第二条:当一个线程访问某对象的synchronized方法或者synchronized代码块时，其他线程仍然可以访问该对象的非同步代码块。

 1 public class MoreThread {
 2     public static void main(String[] args) {
 3         TestSynchronized t = new TestSynchronized();
 4         Thread t1 = new Thread(() -> t.SynMethod());
 5         Thread t2 = new Thread(() -> t.NoSynMethod());
 6 
 7         t1.start();
 8         t2.start();
 9     }
10 
11 }
12 class TestSynchronized {
13 
14     public void SynMethod() {
15         synchronized (this) {
16             try {
17                 for (int i = 0; i < 5; i++) {
18                     Thread.sleep(1000); // 休眠100ms
19                     System.out.println(Thread.currentThread().getName() + " synMethod loop " + i);
20                 }
21             } catch (InterruptedException ie) {
22             }
23         }
24     }
25 
26     public void NoSynMethod() {
27         synchronized (this) {
28             try {
29                 for (int i = 0; i < 5; i++) {
30                     Thread.sleep(1000); // 休眠100ms
31                     System.out.println(Thread.currentThread().getName() + " synMethod loop " + i);
32                 }
33             } catch (InterruptedException e) {
34                 e.printStackTrace();
35             }
36         }
37     }

当线程 t1去调用含有synchronized()的时候,会获取到对象的同步锁（每一个对象有且只有一个同步锁）。此时线程 t2 可以去调用没有synchronized修饰的代码块。不会发生阻塞。执行结果会交叉执行

 1 Thread-0 synMethod loop 0
 2 Thread-1 synMethod loop 0
 3 Thread-0 synMethod loop 1
 4 Thread-1 synMethod loop 1
 5 Thread-0 synMethod loop 2
 6 Thread-1 synMethod loop 2
 7 Thread-1 synMethod loop 3
 8 Thread-0 synMethod loop 3
 9 Thread-1 synMethod loop 4
10 Thread-0 synMethod loop 4
11 
12 Process finished with exit code 0

如果给方法中加上synchroniezd(this) 那么线程t2在执行的时候会发生阻塞。会等一个线程执行完后，释放锁后，另一线程才会运行

 1 Thread-1 synMethod loop 0
 2 Thread-1 synMethod loop 1
 3 Thread-1 synMethod loop 2
 4 Thread-1 synMethod loop 3
 5 Thread-1 synMethod loop 4
 6 Thread-0 synMethod loop 0
 7 Thread-0 synMethod loop 1
 8 Thread-0 synMethod loop 2
 9 Thread-0 synMethod loop 3
10 Thread-0 synMethod loop 4
11 
12 Process finished with exit code 0