synchronized细节问题

一、synchronized有锁重入的特点，某个线程得到对象的锁后，再次请求此对象可以再次得到改对象的锁。如下示例，在method1中调用method2，在method2中调用method3，而method1、method2和method3都是加了synchronized关键字的。

public class SyncDubbo1 {

	public synchronized void method1(){
		System.out.println("method1..");
		method2();
	}
	public synchronized void method2(){
		System.out.println("method2..");
		method3();
	}
	public synchronized void method3(){
		System.out.println("method3..");
	}

	public static void main(String[] args) {
		final SyncDubbo1 sd = new SyncDubbo1();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				sd.method1();
			}
		});
		t1.start();
	}
}

二、父类和子类的方法都是synchronized的，在子类的方法中调用父类的方法，也是线程安全的。

public class SyncDubbo2 {

	static class Main {
		public int i = 10;
		public synchronized void operationSup(){
			try {
				i--;
				System.out.println("Main print i = " + i);
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	static class Sub extends Main {
		public synchronized void operationSub(){
			try {
				while(i > 0) {
					i--;
					System.out.println("Sub print i = " + i);
					Thread.sleep(100);
					this.operationSup();
				}
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	public static void main(String[] args) {

		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				Sub sub = new Sub();
				sub.operationSub();
			}
		});

		t1.start();
	}

}

执行结果：

三、synchronized方法内抛异常怎么处理

锁会自动释放

throw RuntimeException打断此线程或者记录日志然后continue，选择哪种方案取决于具体业务要求。

public class SyncException {

	private int i = 0;
	public synchronized void operation(){
		while(true){
			try {
				i++;
				Thread.sleep(100);
				System.out.println(Thread.currentThread().getName() + " , i = " + i);
				if(i == 20){
					//Integer.parseInt("a");
					throw new RuntimeException();
				}
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	public static void main(String[] args) {

		final SyncException se = new SyncException();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				se.operation();
			}
		},"t1");
		t1.start();
	}

}

四、synchronized代码块锁，实用起来也会比较灵活

　　this、class、Object都可以用来作为代码块锁

public class ObjectLock {

	public void method1(){
		synchronized (this) {	//对象锁
			try {
				System.out.println("do method1..");
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	public void method2(){		//类锁
		synchronized (ObjectLock.class) {
			try {
				System.out.println("do method2..");
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	private Object lock = new Object();
	public void method3(){		//任何对象锁
		synchronized (lock) {
			try {
				System.out.println("do method3..");
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	public static void main(String[] args) {

		final ObjectLock objLock = new ObjectLock();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				objLock.method1();
			}
		});
		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				objLock.method2();
			}
		});
		Thread t3 = new Thread(new Runnable() {
			@Override
			public void run() {
				objLock.method3();
			}
		});

		t1.start();
		t2.start();
		t3.start();

	}

}

五、尽量不要用String常量作为锁

如下代码只会有t1线程运行，但可以new一个String对象。

public class StringLock {

	public void method() {
		//new String("字符串常量")
		synchronized ("字符串常量") {
			try {
				while(true){
					System.out.println("当前线程 : "  + Thread.currentThread().getName() + "开始");
					Thread.sleep(1000);
					System.out.println("当前线程 : "  + Thread.currentThread().getName() + "结束");
				}
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	public static void main(String[] args) {
		final StringLock stringLock = new StringLock();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				stringLock.method();
			}
		},"t1");
		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				stringLock.method();
			}
		},"t2");

		t1.start();
		t2.start();
	}
}

六、一个对象被用作锁时，这个对象内的属性发生变化不会影响锁的使用。

public class ModifyLock {

	private String name ;
	private int age ;

	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}

	public synchronized void changeAttributte(String name, int age) {
		try {
			System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 开始");
			this.setName(name);
			this.setAge(age);

			System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 修改对象内容为： "
					+ this.getName() + ", " + this.getAge());

			Thread.sleep(2000);
			System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 结束");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

	public static void main(String[] args) {
		final ModifyLock modifyLock = new ModifyLock();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				modifyLock.changeAttributte("张三", 20);
			}
		},"t1");
		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				modifyLock.changeAttributte("李四", 21);
			}
		},"t2");

		t1.start();
		try {
			Thread.sleep(100);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		t2.start();
	}

}