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。
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 | 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。
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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 ThreadLocalstringThreadLocal = new ThreadLocal<>();@Testpublic 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。