ThreadLocal 的原理和实例

ThreadLocal 是java开源中非常流行的线程管理工具类，它能防止并发并且能提高效率，应该如果用synchronized,效率实在太差了。
看了它的源码，我们就很容易知道它的原理了：

public class ThreadLocal
{
　private Map values = Collections.synchronizedMap(new HashMap());
　public Object get()
　{
　　Thread curThread = Thread.currentThread(); 
　　Object o = values.get(curThread); 
　　if (o == null && !values.containsKey(curThread))
　　{
　　　o = initialValue();
　　　values.put(curThread, o); 
　　}
　　return o; 
　}

　public void set(Object newValue)
　{
　　values.put(Thread.currentThread(), newValue);
　}

　public Object initialValue()
　{
　　return null; 
　}
} 

注意这个initialValue方法，我们一般是要重写这个方法的。

下面是实例代码。

public class StringThread extends Thread{
    private ThreadLocal tl;
    private String threadName;
    public StringThread(ThreadLocal tl, String threadName){
    	this.tl=tl;
    	this.threadName=threadName;
    }
	@Override
	public void run() {
		if(this.tl!=null){
			for(int i=0;i<3;i++){
				StringBuffer sb=(StringBuffer)this.tl.get();
				sb.append("run");
				System.out.println(this.threadName+":"+this.tl.get());
			}
			
		}
	}
    
}


public class ThreadLocalTest {
	private ThreadLocal tl=new ThreadLocal(){
		@Override
		public Object get() {
			return super.get();
		}
		@Override
		protected Object initialValue() {
			return new StringBuffer();
		}
		@Override
		public void remove() {
			super.remove();
		}
		@Override
		public void set(Object arg0) {
			super.set(arg0);
		}
		
	};
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		ThreadLocalTest tlt=new ThreadLocalTest();
		StringThread st1=new StringThread(tlt.tl,"thread 1 ");
		StringThread st2=new StringThread(tlt.tl,"thread 2 ");
		StringThread st3=new StringThread(tlt.tl,"thread 3 ");
		
		st1.start();
		st2.start();
		st3.start();
	}

}

运行结果是：
thread 3 :run
thread 2 :run
thread 1 :run
thread 2 :runrun
thread 3 :runrun
thread 2 :runrunrun
thread 1 :runrun
thread 3 :runrunrun
thread 1 :runrunrun