1.修饰代码块
    如下，代码块synchronized(object)中是同步的

    1. static int count = 0;
    2. static Object object = new Object();
    3. public void start(){
    4.      for(int i=0;i<10;i++){
    5.          new Thread(()->{
    6.              for(int j=0;j<1000;j++){
    7.                  synchronized(object){
    8.                      count++;
    9.                  }
    10.              }
    11.          }).start();;
    12.     }
    13. }
    14. public static void main(String[] args) throws InterruptedException {
    15.     VolatileVisibilitySample sample = new VolatileVisibilitySample();
    16.     sample.start();
    18.     Thread.sleep(3000);
    19.     System.out.println(count);
    20. }

    2.修饰非静态方法
    如下，方法start中在VolatileVisibilitySample当前的这个对象中是同步的。

    1. public class VolatileVisibilitySample {
    2.     static int count = 0;
    3.     public synchronized void start(){
    4.         for(int j=0;j<1000;j++){
    5.             count++;
    6.         }
    7.     }
    8.     public static void main(String[] args) throws InterruptedException {
    9.         VolatileVisibilitySample sample = new VolatileVisibilitySample();
    11.         for(int i=0;i<10;i++){
    12.             new Thread(()->{
    13.                 sample.start();
    14.             }).start();;
    15.         }
    17.         Thread.sleep(3000);
    18.         System.out.println(count);
    19.     }
    20. }

    注意，修饰非静态方法时，synchronized仅是在当前的类中起到同步作用的，请看下面例子，计算出来的count就是不正确的：

    1. public class VolatileVisibilitySample {
    2.     static int count = 0;
    3.     public synchronized void start(){
    4.         for(int j=0;j<1000;j++){
    5.             count++;
    6.         }
    7.     }
    8.     public static void main(String[] args) throws InterruptedException {
    9.         for(int i=0;i<10;i++){
    10.             new Thread(()->{
    11.                 VolatileVisibilitySample sample = new VolatileVisibilitySample();
    12.                 sample.start();
    13.             }).start();;
    14.         }
    16.         Thread.sleep(3000);
    17.         System.out.println(count);
    18.     }
    19. }

    3.修改静态方法
    如下，静态方法start中是同步的

    1. public class VolatileVisibilitySample {
    2.     static int count = 0;
    3.     public static synchronized void start(){
    4.         for(int j=0;j<1000;j++){
    5.             count++;
    6.         }
    7.     }
    8.     public static void main(String[] args) throws InterruptedException {
    9.         for(int i=0;i<10;i++){
    10.             new Thread(()->{
    11.                 start();
    12.             }).start();;
    13.         }
    15.         Thread.sleep(3000);
    16.         System.out.println(count);
    17.     }
    18. }