ThreadLocal用法详解和原理（转）

本文转自https://www.cnblogs.com/coshaho/p/5127135.html 感谢作者

一、用法

ThreadLocal用于保存某个线程共享变量：对于同一个static ThreadLocal，不同线程只能从中get，set，remove自己的变量，而不会影响其他线程的变量。

1、ThreadLocal.get: 获取ThreadLocal中当前线程共享变量的值。

2、ThreadLocal.set: 设置ThreadLocal中当前线程共享变量的值。

3、ThreadLocal.remove: 移除ThreadLocal中当前线程共享变量的值。

4、ThreadLocal.initialValue: 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。

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package com.coshaho.reflect;   /**  * ThreadLocal用法  * @author coshaho  *  */ 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();                 }             }         }               } <strong>} </strong>

运行结果如下：

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！

线程IntegerTask1: 0

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！

线程IntegerTask2: 0

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！

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！

线程StringTask1: a

线程StringTask2: a

线程StringTask1: aa

线程StringTask2: aa

线程IntegerTask1: 1

线程IntegerTask2: 1

线程StringTask1: aaa

线程StringTask2: aaa

线程IntegerTask2: 2

线程IntegerTask1: 2

线程StringTask2: aaaa

线程StringTask1: aaaa

线程IntegerTask2: 3

线程IntegerTask1: 3

线程StringTask1: aaaaa

线程StringTask2: aaaaa

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！

线程IntegerTask2: 0

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！

线程IntegerTask1: 0

二、原理

线程共享变量缓存如下：

Thread.ThreadLocalMap<ThreadLocal, Object>;

1、Thread: 当前线程，可以通过Thread.currentThread()获取。

2、ThreadLocal：我们的static ThreadLocal变量。

3、Object: 当前线程共享变量。

我们调用ThreadLocal.get方法时，实际上是从当前线程中获取ThreadLocalMap<ThreadLocal, Object>，然后根据当前ThreadLocal获取当前线程共享变量Object。

ThreadLocal.set，ThreadLocal.remove实际上是同样的道理。

关于ThreadLocalMap<ThreadLocal, Object>弱引用问题：

当线程没有结束，但是ThreadLocal已经被回收，则可能导致线程中存在ThreadLocalMap<null, Object>的键值对，造成内存泄露。（ThreadLocal被回收，ThreadLocal关联的线程共享变量还存在）。

虽然ThreadLocal的get，set方法可以清除ThreadLocalMap中key为null的value，但是get，set方法在内存泄露后并不会必然调用，所以为了防止此类情况的出现，我们有两种手段。

1、使用完线程共享变量后，显示调用ThreadLocalMap.remove方法清除线程共享变量；

2、JDK建议ThreadLocal定义为private static，这样ThreadLocal的弱引用问题则不存在了。

-------------------------------

基本原理

线程本地变量是和线程相关的变量，一个线程则一份数据。我们通过ThreadLocal保存的数据最终是保存在Thread类的ThreadLocalMap threadLocals变量中。ThreadlocalMap是一个Map结构，其中key为我们声明的ThreadLocal对象，value即为我们使用ThreadLocal保存的线程本地变量.

当我们调用ThreadLocal变量set方法时，那么为将TheadLocal作为key,set方法的参数做为value保存在当前线程的threadLocals中.调用get方法时类似，调用get方法时，会去Thread的threadLocals中去寻找key为ThreadLocal 变量的值

源码如下：

//Thread.threadLocals变量声明
/* ThreadLocal values pertaining to this thread. This map is maintained
 * by the ThreadLocal class.
 */
ThreadLocal.ThreadLocalMap threadLocals = null;

// ThreadLocal set get方法

/**
 * Sets the current thread's copy of this thread-local variable
 * to the specified value.  Most subclasses will have no need to
 * override this method, relying solely on the {@link #initialValue}
 * method to set the values of thread-locals.
 *
 * @param value the value to be stored in the current thread's copy of
 *        this thread-local.
 */
public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);// getMap方法即去获取当前线程的ThreadLocalMap变量。
    if (map != null)
        map.set(this, value);//以this(ThreadLocal本身)为Key,参数value为值进行保存
    else
        createMap(t, value);
}

/**
 * Get the map associated with a ThreadLocal. Overridden in
 * InheritableThreadLocal.
 *
 * @param  t the current thread
 * @return the map
 */
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

/**
 * Returns the value in the current thread's copy of this
 * thread-local variable.  If the variable has no value for the
 * current thread, it is first initialized to the value returned
 * by an invocation of the {@link #initialValue} method.
 *
 * @return the current thread's value of this thread-local
 */
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();
}

下面是测试代码：

static ThreadLocal<String> stringThreadLocal = new ThreadLocal<>();

@Test
public  void test01(){
    Thread thread1 = new Thread(){
        @Override
        public void run() {
            stringThreadLocal.set("threadName===>"+Thread.currentThread().getName());
            System.out.println(this.getName()+" thread get the value:"+stringThreadLocal.get());

        }
    };
    Thread thread2 = new Thread(){
        @Override
        public void run() {
            stringThreadLocal.set("threadName===>"+Thread.currentThread().getName());
            System.out.println(this.getName()+" thread get the value:"+stringThreadLocal.get());

        }
    };
    Thread thread3 = new Thread(){
        @Override
        public void run() {
            stringThreadLocal.set("threadName===>"+Thread.currentThread().getName());
            System.out.println(this.getName()+" thread get the value:"+stringThreadLocal.get());
        }
    };

    thread1.start();
    thread2.start();
    thread3.start();
    System.out.println("main线程调用set方法之前："+stringThreadLocal.get());
    stringThreadLocal.set("main 线程set的值");
    System.out.println("main线程调用set方法之后："+stringThreadLocal.get());
}

可以看到不同线程设置的值在该线程是能够正确的取到。由于Thread的threadLocals变量只能在Thread所在的包下才能够访问，因此不能对该变量进行直接访问以验证设置的值在Thread.currentThread对象里面。但如果你调试以上代码，设置值之后访问Thread.currentThread.threadLocals会看到之前设置的值。其中key为声明的ThreadLocal对象。