ThreadLocal用法

 

 

1、使用实例：

public class MyThreadLocal {
    private static final ThreadLocal<Object> threadLocal = new ThreadLocal<Object>() {
        /**
         * ThreadLocal没有被当前线程赋值时或当前线程刚调用remove方法后调用get方法，返回此方法值
         */
        @Override
        protected Object initialValue() {
            System.out.println("调用get方法时，当前线程共享变量没有设置，调用initialValue获取默认值！");
            return null;
        }
    };

    public static void main(String[] args) {
        new Thread(new MyIntegerTask("IntegerTask1")).start();
        /*new Thread(new MyStringTask("StringTask1")).start();
        new Thread(new MyIntegerTask("IntegerTask2")).start();
        new Thread(new MyStringTask("StringTask2")).start();*/
    }

    public static class MyIntegerTask implements Runnable {
        private String name;

        MyIntegerTask(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            for (int i = 0; i < 5; i++) {
                // ThreadLocal.get方法获取线程变量
                if (null == MyThreadLocal.threadLocal.get()) {
                    // ThreadLocal.et方法设置线程变量
                    MyThreadLocal.threadLocal.set(0);
                    System.out.println("线程" + name + ": 0");
                } else {
                    int num = (Integer) MyThreadLocal.threadLocal.get();
                    MyThreadLocal.threadLocal.set(num + 1);
                    System.out.println("线程" + name + ": " + MyThreadLocal.threadLocal.get());
                    if (i == 3) {
                        MyThreadLocal.threadLocal.remove();
                    }
                }
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }

    }

    public static class MyStringTask implements Runnable {
        private String name;

        MyStringTask(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            for (int i = 0; i < 5; i++) {
                if (null == MyThreadLocal.threadLocal.get()) {
                    MyThreadLocal.threadLocal.set("a");
                    System.out.println("线程" + name + ": a");
                } else {
                    String str = (String) MyThreadLocal.threadLocal.get();
                    MyThreadLocal.threadLocal.set(str + "a");
                    System.out.println("线程" + name + ": " + MyThreadLocal.threadLocal.get());
                }
                try {
                    Thread.sleep(800);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }

    }
}

　　2、源码简要分析：

　　

测试：
public class MyThreadLocal {                                                              
    private static final ThreadLocal<Object> threadLocal = new ThreadLocal<Object>();       
    public static void main(String[] args) {
     new Thread(new MyIntegerTask("IntegerTask1")).start();
  }
  public static class MyIntegerTask implements Runnable {
      @Override
        public void run() {
            if (null == MyThreadLocal.threadLocal.get()) {
              MyThreadLocal.threadLocal.set(0);
              System.out.println("线程本地值为： 0");
           }else {
              int num = (Integer) MyThreadLocal.threadLocal.get();
              MyThreadLocal.threadLocal.set(1);
              System.out.println("线程本地值为: " + MyThreadLocal.threadLocal.get());
           }
          if (i == 1) {
              MyThreadLocal.threadLocal.remove();
           }
        }
  }
}

ThreadLocal类的代码片段：
public class ThreadLocal<T> {
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }
    
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
    
     public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }
     
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
   
    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;
    }
    
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }
         
    static class ThreadLocalMap {
        ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }    
    }
}

public  class Thread implements Runnable {
     ThreadLocal.ThreadLocalMap threadLocals = null;
}