java 中ThreadLocal实例分析
java 中ThreadLocal实例分析
从概念上理解,threadlocal使变量在多个线程中相互隔离实现线程安全,threadlocal包装的变量最终都专属于对应的每个线程,线程之间相互独立,用一个具体实现来说明:
publicinterfaceConsumer{ intconsume(); } publicclassComsumeThreadimplementsRunnable{ privateConsumerconsumer; publicComsumeThread(Consumerconsumer){ this.consumer=consumer; } @Override publicvoidrun(){ for(inti=0;i<10;i++){ System.out.println(Thread.currentThread().getName()+"AfterConsumeleft:"+consumer.consume()); } } } publicclassConsumeClientAimplementsConsumer{ privatestaticintleftNum=30; @Override publicintconsume(){ intorgLeftNum=leftNum; Randomrandom=newRandom(System.currentTimeMillis()); try{ Thread.sleep(random.nextInt(3)); }catch(InterruptedExceptione){ e.printStackTrace(); } orgLeftNum=orgLeftNum-1; leftNum=orgLeftNum; returnleftNum; } publicstaticvoidmain(String[]args){ Consumerconsumer=newConsumeClientA(); Threadthread1=newThread(newComsumeThread(consumer)); Threadthread2=newThread(newComsumeThread(consumer)); Threadthread3=newThread(newComsumeThread(consumer)); thread1.start(); thread2.start(); thread3.start(); } }
ConsumeClientA是在没有做任何线程安全处理,结果如下:
Thread-2AfterConsumeleft:29 Thread-1AfterConsumeleft:29 Thread-3AfterConsumeleft:29 Thread-2AfterConsumeleft:28 Thread-1AfterConsumeleft:28 Thread-3AfterConsumeleft:28 Thread-2AfterConsumeleft:27 Thread-1AfterConsumeleft:27 Thread-2AfterConsumeleft:26 Thread-3AfterConsumeleft:27 Thread-1AfterConsumeleft:25 Thread-2AfterConsumeleft:25 Thread-3AfterConsumeleft:25 Thread-1AfterConsumeleft:24 Thread-2AfterConsumeleft:24 Thread-3AfterConsumeleft:24 Thread-1AfterConsumeleft:23 Thread-2AfterConsumeleft:23 Thread-3AfterConsumeleft:23 Thread-1AfterConsumeleft:22 Thread-2AfterConsumeleft:22 Thread-3AfterConsumeleft:22 Thread-1AfterConsumeleft:21 Thread-2AfterConsumeleft:21 Thread-3AfterConsumeleft:21 Thread-1AfterConsumeleft:20 Thread-2AfterConsumeleft:20 Thread-3AfterConsumeleft:20 Thread-1AfterConsumeleft:19 Thread-3AfterConsumeleft:18
增加threadlocal处理,每个线程相互独立,实现如下:
publicclassConsumeClientBimplementsConsumer{ privateThreadLocalleftNumThreadLocal=newThreadLocal (){ @Override protectedIntegerinitialValue(){ return30; } }; @Override publicintconsume(){ intorgLeftNum=leftNumThreadLocal.get(); Randomrandom=newRandom(System.currentTimeMillis()); try{ Thread.sleep(random.nextInt(3)); }catch(InterruptedExceptione){ e.printStackTrace(); } orgLeftNum=orgLeftNum-1; leftNumThreadLocal.set(orgLeftNum); returnleftNumThreadLocal.get(); } publicstaticvoidmain(String[]args){ Consumerconsumer=newConsumeClientB(); Threadthread1=newThread(newComsumeThread(consumer)); Threadthread2=newThread(newComsumeThread(consumer)); Threadthread3=newThread(newComsumeThread(consumer)); thread1.start(); thread2.start(); thread3.start(); } }
运行的结果如下:
Thread-1AfterConsumeleft:29 Thread-3AfterConsumeleft:29 Thread-2AfterConsumeleft:29 Thread-1AfterConsumeleft:28 Thread-3AfterConsumeleft:28 Thread-2AfterConsumeleft:28 Thread-1AfterConsumeleft:27 Thread-3AfterConsumeleft:27 Thread-2AfterConsumeleft:27 Thread-1AfterConsumeleft:26 Thread-3AfterConsumeleft:26 Thread-2AfterConsumeleft:26 Thread-1AfterConsumeleft:25 Thread-3AfterConsumeleft:25 Thread-2AfterConsumeleft:25 Thread-1AfterConsumeleft:24 Thread-3AfterConsumeleft:24 Thread-2AfterConsumeleft:24 Thread-1AfterConsumeleft:23 Thread-3AfterConsumeleft:23 Thread-2AfterConsumeleft:23 Thread-1AfterConsumeleft:22 Thread-3AfterConsumeleft:22 Thread-2AfterConsumeleft:22 Thread-1AfterConsumeleft:21 Thread-3AfterConsumeleft:21 Thread-2AfterConsumeleft:21 Thread-1AfterConsumeleft:20 Thread-3AfterConsumeleft:20 Thread-2AfterConsumeleft:20
每个线程拥有自己的独立变量,相互隔离实现线程安全。
那ThreadLocal是怎样实现这种线程隔离的线程安全的呢?
从ThreadLocal源码可以看到,真正实现线程隔离,与线程挂钩的,其实是ThreadLocal.ThreadLocalMap这个实现类,最明显的体现就在于Thread类源码的这样一个变量申明说明了ThreadLocal.ThreadLocalMap与Thread的关系:
ThreadLocal.ThreadLocalMapthreadLocals,inheritableThreadLocals;
Thread类是包含threadLocals对象的,ThreadLocal的具体实现就是根据提供的get,set等接口,对当前thread的threadLocals变量进行相关操作的,如get操作代码如下:
publicTget(){ Threadt=Thread.currentThread(); ThreadLocalMapmap=getMap(t); if(map!=null){ ThreadLocalMap.Entrye=map.getEntry(this); if(e!=null) return(T)e.value; } returnsetInitialValue(); } ThreadLocal.ThreadLocalMapgetMap(Threadt){ returnt.threadLocals; }
可以看到,getMap()方法就是从当前thread获取对应的threadLocals变量,然后从这个ThreadLocal.ThreadLocalMap类型的threadLocals变量中获取对应线程中该ThreadLocal对象对应的变量值。
set方法的操作也是一样:
publicvoidset(Tvalue){ Threadt=Thread.currentThread(); ThreadLocal.ThreadLocalMapmap=getMap(t); if(map!=null){ map.set(this,value); }else{ this.createMap(t,value); } } voidcreateMap(Threadt,TfirstValue){ t.threadLocals=newThreadLocalMap(this,firstValue); }
staticclassEntryextendsWeakReference{ Objectvalue; Entry(ThreadLocalvar1,Objectvar2){ super(var1); this.value=var2; } }
ThreadLocalMap中存的是内部类Entry的数组,Entry是继承WeakReference实现,WeakReference的好处是保存对象引用,而又不干扰该对象被GC回收,线程执行完回收threadLocals变量时不会受到Entry封装的变量的干扰。
而且ThreadLocalMap中的key是ThreadLocal,所以一个ThreadLocal对象只能在一个Thread对象中保存一个ThreadLocal的value。
综上,很多人说ThreadLocal的实现是ThreadLocalMap中存Thread对象为key,变量为value的map结构,其实是错误的。
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