Java 并发编程基础 --Java 内存模型

public class VolatileTest {
	public static volatile int race = 0;
	public static void increase() {
		race++;
	}
	public static void main(String[] args) {
		Thread[] threads = new Thread[20];
		for (int i = 0; i < 20; i++) {
			threads[i] = new Thread(() -> {
				for (int j = 0; j < 1000; j++) {
					increase();
				}
			});
			threads[i].start();
		}
		while (Thread.activeCount() > 1) {
			Thread.yield();
		}
		System.out.println(race);
	}
}

public class com.concurrent.VolatileTest {
  public static volatile int race;
  public com.concurrent.VolatileTest();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return
  public static void increase();
    Code:
       0: getstatic     #2                  // Field race:I
       3: iconst_1
       4: iadd
       5: putstatic     #2                  // Field race:I
       8: return
  public static void main(java.lang.String[]);
    Code:
       0: bipush        20
       2: anewarray     #3                  // class java/lang/Thread
       5: astore_1
       6: iconst_0
       7: istore_2
       8: iload_2
       9: bipush        20
      11: if_icmpge     41
      14: aload_1
      15: iload_2
      16: new           #3                  // class java/lang/Thread
      19: dup
      20: invokedynamic #4,  0              // InvokeDynamic #0:run:()Ljava/lang/Runnable;
      25: invokespecial #5                  // Method java/lang/Thread."<init>":(Ljava/lang/Runnable;)V
      28: aastore
      29: aload_1
      30: iload_2
      31: aaload
      32: invokevirtual #6                  // Method java/lang/Thread.start:()V
      35: iinc          2, 1
      38: goto          8
      41: invokestatic  #7                  // Method java/lang/Thread.activeCount:()I
      44: iconst_1
      45: if_icmple     54
      48: invokestatic  #8                  // Method java/lang/Thread.yield:()V
      51: goto          41
      54: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
      57: getstatic     #2                  // Field race:I
      60: invokevirtual #10                 // Method java/io/PrintStream.println:(I)V
      63: return
  static {};
    Code:
       0: iconst_0
       1: putstatic     #2                  // Field race:I
       4: return
}

volatile boolean shutDownRequested;
public void shutdown() {
	shutDownRequested = true;
}
public void doWork() {
	while (!shutDownRequested) {
		// do something
	}
}

Map configOptions;
char[] configText;
// 此变量必须是volatile
volatile boolean initialized= false;
// 假设下面代码在线程A中执行
// 模拟读取配置信息，当读取完成后将initialized设置为true以通知其他线程配置可用
configOptions = new HashMap();
configText = readConfigFile(fileName);
processConfigOptions(configText, configOptions);
initialized= true;
// 假设下面代码在线程B中执行
// 等待initlizated 为true，代表线程A已经把配置信息初始化完成
while (!initialized) {
	sleep();
}
// 使用线程A中初始化好的配置
doSomethingWithConfig();

public class LazyDoubleCheckSingleton {
	private static volatile LazyDoubleCheckSingleton instance;
	private LazyDoubleCheckSingleton() {}
	public static LazyDoubleCheckSingleton getInstance() {
		if (instance == null) {
			synchronized (LazyDoubleCheckSingleton.class) {
				if (instance == null) {
					instance = new LazyDoubleCheckSingleton();
				}
			}
		}
		return instance;
	}
}

public class com.designpattern.singleton.LazyDoubleCheckSingleton {
  public static com.designpattern.singleton.LazyDoubleCheckSingleton getInstance();
    Code:
       0: getstatic     #2                  // Field instance:Lcom/designpattern/singleton/LazyDoubleCheckSingleton;
       3: ifnonnull     37
       6: ldc           #3                  // class com/designpattern/singleton/LazyDoubleCheckSingleton
       8: dup
       9: astore_0
      10: monitorenter
      11: getstatic     #2                  // Field instance:Lcom/designpattern/singleton/LazyDoubleCheckSingleton;
      14: ifnonnull     27
      17: new           #3                  // class com/designpattern/singleton/LazyDoubleCheckSingleton
      20: dup
      21: invokespecial #4                  // Method "<init>":()V
      24: putstatic     #2                  // Field instance:Lcom/designpattern/singleton/LazyDoubleCheckSingleton;
      27: aload_0
      28: monitorexit
      29: goto          37
      32: astore_1
      33: aload_0
      34: monitorexit
      35: aload_1
      36: athrow
      37: getstatic     #2                  // Field instance:Lcom/designpattern/singleton/LazyDoubleCheckSingleton;
      40: areturn
    Exception table:
       from    to  target type
          11    29    32   any
          32    35    32   any
}