多线程编程-线程间通信-ThreadLocal（七）

叁歲伎倆 2022-06-01 02:54 258阅读 0赞

1，ThreadLocal类主要解决的是每个线程绑定自己的值，这里存储的是每个线程私有的数据，解决了变量在不同线程间的隔离性。

测试代码：

public class Tools {
    	public static ThreadLocal tl = new ThreadLocal();
    }

public class ThreadA extends Thread {
    
    	@Override
    	public void run() {
    		try {
    			for(int i=0; i<10; i++){
    				Tools.tl.set("ThreadA "+(i+1));
    				System.out.println("ThreadA get value="+Tools.tl.get());
    			}
    			Thread.sleep(2000);
    		} catch (InterruptedException e) {
    			e.printStackTrace();
    		}
    	}
    }

public class ThreadB extends Thread {
    
    	@Override
    	public void run() {
    		try {
    			for(int i=0; i<10; i++){
    				Tools.tl.set("ThreadB "+(i+1));
    				System.out.println("ThreadB get value="+Tools.tl.get());
    			}
    			Thread.sleep(2000);
    		} catch (InterruptedException e) {
    			e.printStackTrace();
    		}
    	}
    }

public class RunTest {
    
    	public static void main(String[] args) {
    		ThreadA ta = new ThreadA();
    		ThreadB tb = new ThreadB();
    		ta.start();
    		tb.start();
    	}
    }

运行结果：

ThreadB get value=ThreadB 1  
ThreadA get value=ThreadA 1  
ThreadA get value=ThreadA 2  
ThreadA get value=ThreadA 3  
ThreadA get value=ThreadA 4  
ThreadA get value=ThreadA 5  
ThreadA get value=ThreadA 6  
ThreadA get value=ThreadA 7  
ThreadA get value=ThreadA 8  
ThreadA get value=ThreadA 9  
ThreadA get value=ThreadA 10  
ThreadB get value=ThreadB 2  
ThreadB get value=ThreadB 3  
ThreadB get value=ThreadB 4  
ThreadB get value=ThreadB 5  
ThreadB get value=ThreadB 6  
ThreadB get value=ThreadB 7  
ThreadB get value=ThreadB 8  
ThreadB get value=ThreadB 9  
ThreadB get value=ThreadB 10  
  
每个线程都取出了自己设置的值。

2，ThreadLocal第一次get会返回null，可以通过重载其方法initialValue来设置默认值。

测试代码：

public class Tools {
    	public static ThreadLocalExt tl = new ThreadLocalExt();
    }
    class ThreadLocalExt extends ThreadLocal{
    
    	@Override
    	protected Object initialValue() {
    		System.out.println("default value not null.");
    		return new Date().getTime();
    	}
    }

public class ThreadA extends Thread {
    
    	@Override
    	public void run() {
    		try {
    			Thread.sleep(2000);
    			System.out.println("ThreadA get value="+Tools.tl.get());
    		} catch (InterruptedException e) {
    			e.printStackTrace();
    		}
    	}
    }

public class ThreadB extends Thread {
    
    	@Override
    	public void run() {
    		try {
    			Thread.sleep(8000);
    			System.out.println("ThreadB get value="+Tools.tl.get());
    		} catch (InterruptedException e) {
    			e.printStackTrace();
    		}
    	}
    }

public class RunTest {
    
    	public static void main(String[] args) {
    		ThreadA ta = new ThreadA();
    		ThreadB tb = new ThreadB();
    		ta.start();
    		tb.start();
    		if(Tools.tl.get() == null){
    			System.out.println("main thread 还没有设置过值");
    			Tools.tl.set(new Date().getTime());
    		}
    		System.out.println("main thread ,tl="+Tools.tl.get());
    	}
    }

运行结果：

default value not null.  
main thread ,tl=1516241174168  
default value not null.  
ThreadA get value=1516241176167  
default value not null.  
ThreadB get value=1516241182168  
  
每个线程都有自己的初始值。

3，InheritableThreadLocal的使用

类InheritableThreadLocal，可以让子线程取得从父线程继承下来的。

测试代码：

public class Tools {
    	public static InheritableThreadLocalExt tl = new InheritableThreadLocalExt();
    }
    class InheritableThreadLocalExt extends InheritableThreadLocal{
    	@Override
    	protected Object initialValue() {
    		System.out.println("default value not null.");
    		return new Date().getTime();
    	}
    
    	@Override
    	protected Object childValue(Object parentValue) {
    		System.out.println("child value inheritable.");
    		return parentValue+"-child change value";
    	}
    	
    }

public class RunTest {
    
    	public static void main(String[] args) {
    		if(Tools.tl.get() == null){
    			System.out.println("main thread 还没有设置过值");
    			Tools.tl.set(new Date().getTime());
    		}
    		System.out.println("main thread ,tl="+Tools.tl.get());
    		ThreadA ta = new ThreadA();
    		ThreadB tb = new ThreadB();	
    		ta.start();
    		tb.start();		
    	}
    }

ThreadA，ThreadB的代码不变，但是RunTest.java代码稍微有改动，就是把ThreadA，ThreadB的对象创建放在了主线程的这句：System.out.println("main thread ,tl="+Tools.tl.get());之后。如果把线程A，B的创建放在这句调用之前，A，B线程不会继承父线程（主线程）的值。

protected Object childValue(Object parentValue) \{\}是在继承的同时，可以对值做修改。

运行结果：

default value not null.  
main thread ,tl=1516245837214  
child value inheritable.  
child value inheritable.  
ThreadA get value=1516245837214-child change value  
ThreadB get value=1516245837214-child change value

可以看到，虽然线程A，B都有几秒的延迟，但是获取的时间都是main线程的InheritableThreadLocal的时间。