本文共 6424 字，大约阅读时间需要 21 分钟。

ThreadLocal

• 为解决多线程并发问题提供一种新思路。ThreadLocal并不是一个Thread，而是Thread局部变量

ThreadLocal作用

• 解决多线程环境下整个上下文调用需要将关键参数透传
• 如果不使用ThreadLocal，每个方法都要加关键参数，如果内部方法链路过长，那么代码看起来冗余、臃肿
• 如果某处传时将参数值改掉或设置为null，后续调用方法中用到这个参数的代码会受到影响
• 未用ThreadLocal的例子，模拟HTTP服务器接收用户请求

  package com.myd.cn.ThreadLocal;	import java.util.Collections;	import java.util.List;	import java.util.concurrent.ExecutorService;	import java.util.concurrent.Executors;	public class NoThreadLocalHTTPServer {		public static void main(String[] args) {			simpleTest();		}		public static void simpleTest(){			ExecutorService executorService = Executors.newFixedThreadPool(2);			//提交线程任务			executorService.submit(()->{				Long userId = 10000L;				doUserRequest(userId);			});			executorService.submit(()->{				Long userId = 20000L;				doUserRequest(userId);			});		}		private static void doUserRequest(Long userId) {			ThreadContext threadContext = new ThreadContext();			threadContext.setUserId(userId);			String info = getCusInfo(threadContext);			List
       
         courses = getMyCourse(threadContext);			System.out.println("user info "+info +";courses = "+courses.toString());		}		private static String getCusInfo(ThreadContext threadContext){			return threadContext.getUserId() +" 的UserId信息";		}		private static List
        
          getMyCourse(ThreadContext threadContext){			return Collections.singletonList(threadContext.getUserId()+" 相应的课程");		}		public static class ThreadContext{			private Long userId;			public Long getUserId() {				return userId;			}			public void setUserId(Long userId) {				this.userId = userId;			}		}	}
        
       

• 使用ThreadLocal例子

  package com.myd.cn.ThreadLocal;	import java.util.Collections;	import java.util.List;	import java.util.concurrent.ExecutorService;	import java.util.concurrent.Executors;	public class ThreadLocalHTTPServer {		public static void main(String[] args) {			simpleTest();		}		private static void simpleTest() {			ExecutorService executorService = Executors.newFixedThreadPool(2);			executorService.submit(()->{				 Long userId = 1000L;				 doUseRequeset(userId);			});			executorService.submit(()->{				 Long userId = 2000L;				 doUseRequeset(userId);			});		}				private static void doUseRequeset(Long userId) {			UserSessionContext.getUserSessionContext().setUserId(userId);			String info = getCusInfo();			List
       
         courses = getCourse();			System.out.println("user info "+info +";courses = "+courses.toString());		}		public static String getCusInfo(){			return UserSessionContext.getUserSessionContext().getUserId()+" 的相关信息";		}		public static List
        
          getCourse(){			return Collections.singletonList(UserSessionContext.getUserSessionContext().getUserId()+"的 相关课程信息");		}		private static class UserSessionContext{			private static ThreadLocal
         
           threadLocal = ThreadLocal.withInitial(UserSessionContext::new);			//获取ThreadLocal包含的（线程本地存储)的变量UserSessionContext			public static UserSessionContext getUserSessionContext(){				return threadLocal.get();			}			/**			 * 删除本地存储的变量，线程执行完毕时删除，避免内存溢出			 */			public static void removeContext(){				threadLocal.remove();			}			private Long userId;			public static ThreadLocal
          
            getThreadLocal() {				return threadLocal;			}			public static void setThreadLocal(ThreadLocal
           
             threadLocal) {				UserSessionContext.threadLocal = threadLocal;			}			public Long getUserId() {				return userId;			}			public void setUserId(Long userId) {				System.out.println(Thread.currentThread().getName()+ " set userId = "+userId);				this.userId = userId;			}		}	}
           
          
         
        
       

ThreadLocal原理

• java四种引用方式
  
• 弱引用例子：对象被弱引用后置空，JVM触发GC会将其回收

1.代码

import java.lang.ref.WeakReference;		/**		 * 测试弱引用，弱引用的对象并没有被其他对象直接引用，		 * 那么对于ThreadLocal->threadlocals->threadLlocalMap->key的弱引用被回收，相应强引用的value则不会被回收，造成OOM		 * @author dymll		 *		 */		public class WeakReferenceOfThreadLocal {			public static void main(String[] args) throws Exception {				testWeakReferenceNoDirectReference() ;			}			public static void testWeakReferenceNoDirectReference() throws Exception{				Object obj = new Object();				WeakReference

2.结果，查看被list对象引用obj仍有输出

• 如果对象被弱引用，且其后又被其他对象直接引用(强引用),则后面即使被置空，也不会被触发的GC操作回收掉

  1.代码		package com.myd.cn.ThreadLocal;		import java.lang.ref.WeakReference;		import java.util.ArrayList;		import java.util.List;		/**		 * 测试弱引用，弱引用的对象并没有被其他对象直接引用，		 * 那么对于ThreadLocal->threadlocals->threadLlocalMap->key的弱引用被回收，相应强引用的value则不会被回收，造成OOM		 * @author dymll		 *		 */		public class StrongReferenceAfterWeakReference {			public static void main(String[] args) throws Exception {				testWeakReferenceNoDirectReference() ;			}			public static void testWeakReferenceNoDirectReference() throws Exception{				Object obj = new Object();				WeakReference

2.结果，查看被list对象引用obj仍有输出

## ThreadLocal数据结构- 查看其源码，ThreadLocal数据存在其下Threadlocals的ThreadLocalMap,Map有Entry构成，Entry包含每个线程作为key，value是每个线程专有的数据(这里是变量)	![ThreadLocal数据结构](https://img-blog.csdnimg.cn/20200327000458779.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2R5bWtrag==,size_16,color_FFFFFF,t_70)- 每个线程都有自己的ThreadLocal	![每个线程都有自己ThreadLocal](https://img-blog.csdnimg.cn/20200327000515974.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2R5bWtrag==,size_16,color_FFFFFF,t_70)## ThreadLocal OOM问题- key是软引用，被置空后会被回收，那么Thread->ThreadLocalMap->Entry->Value这个链路是强引用链路，ThreadLocalMap无法再通过key来访问value，value在这里不会被回收，当这样的对象过多占用内存是，发生OOM	![强引用且无法进行回收，而不能进行内存释放，造成内存泄漏](https://img-blog.csdnimg.cn/20200327000544676.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2R5bWtrag==,size_16,color_FFFFFF,t_70)- 如果解决ThreadLocal OOM问题- 在使用ThreadLocal 时，都要在线程全部指向完毕后，在finally代码块中调用remove()方法,清除内存(线程池中尤为注意要清除内存，更易因为强引用无法回收内存，造成OOM）- 保存在ThreadLocal的数据不能过大

转载地址：http://ihjzb.baihongyu.com/