ThreadLocal父子间通信的四种解决方案
ThreadLocal 是存储在线程栈帧中的一块数据存储区域，其可以做到线程与线程之间的读写隔离。
但是在我们的日常场景中，经常会出现父线程需要向子线程中传递消息，而 ThreadLocal 仅能在当前线程上进行数据缓存，这里就介绍4种父子间通信问题;
- 在子线程中手动设置父线程的值
- ThreadPoolTaskExecutor + TaskDecorator
- InheritableThreadLocal
- TransmittableThreadLocal
1.在子线程中手动设置父线程的值
ThreadLocal<String> threadLocal = new ThreadLocal<>();
@BeforeEach
public void init() {
threadLocal.set("thread-01");
}
@Test
public void test4() {
String s = threadLocal.get();
new Thread(() -> {
threadLocal.set(s);
System.out.println(threadLocal.get());
}).start();
threadLocal.remove();
}
在子线程里手动的设置变量,@BeforeEach是junit5的写法,对应junit4的Before
输出结果: thread-01
2.ThreadPoolTaskExecutor + TaskDecorator
使用ThreadPoolTaskExecutor线程池的时候,可自定义一个TaskDecorator包装类,这个类的作用就是在执行子线程之前手动的设置父线程的变量,跟第一种方法类似;
- 储存线程用户信息
public class UserContextUtils {
private static final ThreadLocal<String> userThreadLocal = new ThreadLocal<>();
public static void set(String username) {
userThreadLocal.set(username);
}
public static String get() {
return userThreadLocal.get();
}
public static void clear() {
userThreadLocal.remove();
}
}
- 这是一个执行回调方法的装饰器，主要应用于传递上下文，或者提供任务的监控/统计信息
public class ContextTaskDecorator implements TaskDecorator {
@Override
public Runnable decorate(Runnable runnable) {
String username = UserContextUtils.get();
return () -> {
try {
// 将主线程的请求信息，设置到子线程中
UserContextUtils.set(username);
// 执行子线程，这一步不要忘了
runnable.run();
} finally {
// 线程结束，清空这些信息，否则可能造成内存泄漏
UserContextUtils.clear();
}
};
}
}
- 初始化线程池
@Bean
public Executor getAsyncExecutor() {
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
executor.setCorePoolSize(4);
executor.setMaxPoolSize(8);
executor.setQueueCapacity(100);
executor.setAllowCoreThreadTimeOut(false);
executor.setKeepAliveSeconds(0);
executor.setThreadNamePrefix("DefaultAsync-");
executor.setTaskDecorator(new ContextTaskDecorator());
executor.setWaitForTasksToCompleteOnShutdown(true);
executor.initialize();
System.out.println("初始化Async的线程池");
return executor;
}
- 测试
@BeforeEach
public void init() {
threadLocal.set("thread-01");
UserContextUtils.set("taskExecutor");
}
@Resource
private ThreadPoolTaskExecutor getAsyncExecutor;
@Test
public void test3() {
getAsyncExecutor.execute(()->{
System.out.println(UserContextUtils.get());
});
}
使用ThreadPoolTaskExecutor线程池的时候，使用构造器在子线程写入主线程参数,但是使用ThreadPoolExecutor就不能这么做了,建议使用第四种方式TTL;
3.InheritableThreadLocal
inheritableThreadLocal是ThreadLocal中自带的一种方法,只要替换原来的ThreadLocal就行了,但是这种方法有缺陷,会存在核心线程旧值的重复使用,不建议使用;
这里我设置一个线程池,核心线程数为2个,核心线程数重复使用的时候不会重新拿新值,而是用原来的旧值
@Test
public void test1() {
//1.创建一个自己定义的线程池
ExecutorService executorService = new ThreadPoolExecutor(2, 3, 0, TimeUnit.MILLISECONDS, new SynchronousQueue<>());
InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
threadLocal.set("thread-01");
executorService.execute(() -> {
String s = threadLocal.get();
System.out.println(s);
});
threadLocal.set("thread-02");
executorService.execute(() -> {
String s = threadLocal.get();
System.out.println(s);
});
threadLocal.set("thread-03");
executorService.execute(() -> {
String s = threadLocal.get();
System.out.println(s);
});
threadLocal.set("thread-04");
executorService.execute(() -> {
String s = threadLocal.get();
System.out.println(s);
});
}
测试结果: 因为线程2被重复使用
thread-01
thread-02
thread-02
thread-02
4.TransmittableThreadLocal
TransmittableThreadLocal 是Alibaba开源的、用于解决 “在使用线程池等会缓存线程的组件情况下传递ThreadLocal” 问题的 InheritableThreadLocal 扩展。若希望 TransmittableThreadLocal 在线程池与主线程间传递，需配合 TtlExecutors.getTtlExecutorService,TtlRunnable 和 TtlCallable 使用。
- 引入TTL的jar包
<dependency>
<groupId>com.alibaba</groupId>
<artifactId>transmittable-thread-local</artifactId>
<version>2.12.6</version>
</dependency>
- TtlExecutors.getTtlExecutorService()
使用Ttl提供的TtlExecutors.getTtlExecutorService来对原来线程池进行包装,但是此时变量需要使用TransmittableThreadLocal,建议使用这种方式;
@Test
public void test2() {
// 1. 创建一个自己定义的线程池
ExecutorService executorService = new ThreadPoolExecutor(2, 3, 0, TimeUnit.MILLISECONDS, new SynchronousQueue<>());
// 2. 使用TransmittableThreadLocal修饰变量
TransmittableThreadLocal<String> threadLocal1 = new TransmittableThreadLocal<>();
// 3. 使用TtlExecutors.getTtlExecutorService包装线程池
ExecutorService ttlExecutorService = TtlExecutors.getTtlExecutorService(executorService);
threadLocal1.set("thread-01");
ttlExecutorService.execute(() -> {
String s = threadLocal1.get();
System.out.println(s);
});
threadLocal1.set("thread-02");
ttlExecutorService.execute(() -> {
String s = threadLocal1.get();
System.out.println(s);
});
threadLocal1.set("thread-03");
ttlExecutorService.execute(() -> {
String s = threadLocal1.get();
System.out.println(s);
});
threadLocal1.set("thread-04");
ttlExecutorService.execute(() -> {
String s = threadLocal1.get();
System.out.println(s);
});
}
thread-01
thread-02
thread-03
thread-04
- TtlRunnable.get()
@Test
public void test5() {
// 1. 创建一个自己定义的线程池
ExecutorService executorService = new ThreadPoolExecutor(2, 3, 2, TimeUnit.MILLISECONDS, new SynchronousQueue<>());
// 2. 使用TransmittableThreadLocal修饰变量
TransmittableThreadLocal<String> threadLocal1 = new TransmittableThreadLocal<>();
threadLocal1.set("thread-01");
executorService.execute(TtlRunnable.get(() -> {
String s = threadLocal1.get();
System.out.println(s);
}));
threadLocal1.set("thread-02");
executorService.execute(TtlRunnable.get(() -> {
String s = threadLocal1.get();
System.out.println(s);
}));
threadLocal1.set("thread-03");
executorService.execute(TtlRunnable.get(() -> {
String s = threadLocal1.get();
System.out.println(s);
}));
threadLocal1.set("thread-04");
executorService.execute(TtlRunnable.get(() -> {
String s = threadLocal1.get();
System.out.println(s);
}));
}
thread-01
thread-02
thread-03
thread-04