1.概述

• 线程的局部变量, 每个线程自己独立隔离开来,线程之间的数据不共享,所以不会有数据安全性问题.
• ThreadLocal，很多地方叫做线程本地变量，也有些地方叫做线程本地存储，其实意思差不多。ThreadLocal为变量在每个线程中都创建了一个副本，那么每个线程可以访问自己内部的副本变量。

2.示例

public class ThreadLocalTest {

	private ThreadLocal<Integer> count = new ThreadLocal<Integer>() {
		protected Integer initialValue() {
			return new Integer(0);
		};
	};

	public int getNext() {
		Integer value = count.get();
		value++;
		count.set(value);
		return value;
	}

	public static void main(String[] args) {
		ThreadLocalTest threadLocalTest = new ThreadLocalTest();

		new Thread(new Runnable() {
			@Override
			public void run() {
				while (true) {
					System.out.println(Thread.currentThread().getName() + " " + threadLocalTest.getNext());
					try {
						Thread.sleep(2000);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}
		}).start();

		new Thread(new Runnable() {
			@Override
			public void run() {
				while (true) {
					System.out.println(Thread.currentThread().getName() + " " + threadLocalTest.getNext());
					try {
						Thread.sleep(500);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}
		}).start();
	}

}

3.ThreadLocal源码简析

public class ThreadLocal<T> {
    //1.获取值
    public T get() {
        Thread t = Thread.currentThread();  //当前线程
        ThreadLocalMap map = getMap(t);     //当前线程是否在ThreadLocalMap中
        if (map != null) {      //如有ThreadLocalMap,则取值
            ThreadLocalMap.Entry e = map.getEntry(this);//entry是map中的组(包含key和value)
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        //为空,返回初始值结果
        return setInitialValue();
    }
    
    //2.设置值
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
    
    //3.去掉值
     public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }
     
     
     //4.设置初始值
    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }


    //5.获取map中的值,将绑定到Thread的threadLocals中.
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
    
    
    //6.创建一个createMap
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }
    
    
        //ThreadLocal的内部类ThreadLocalMap,将key(线程)和值存在Entry中
    static class ThreadLocalMap {
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }
        
        private void set(ThreadLocal<?> key, Object value) {
            ....      
        }
        
         private void remove(ThreadLocal<?> key) {
             ....  
        }
        
        private Entry getEntry(ThreadLocal<?> key) {
        }
        
    }
}


//Thread中有ThreadLocal.ThreadLocalMap,存的是thread真正的值
public class Thread implements Runnable {
    ThreadLocal.ThreadLocalMap threadLocals = null;
}

• 将当前线程和当前线程计算的值保存到ThreadLocalMap对象的Entry中(ThreadLocal为key,value为值),
• 而ThreadLocalMap对象又存在放当前Thread类下的ThreadLocal.ThreadLocalMap中. 取值的时候到当前线程中取ThreadLocal.ThreadLocalMap的Entry响应的值就可以了.

Entry(ThreadLocal,Object)=>Entry存到ThreadLocalMap下=>Thread.ThreadLocal.ThreadLocalMap 存储了 ThreadLocalMap