Java多线程父线程向子线程传值解决方案

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1 背景

在实际开发过程中我们需要父子之间传递一些数据，比如用户信息，日志异步生成数据传递等，该文章从5种解决方案解决父子之间数据传递困扰

2 ThreadLocal+TaskDecorator

用户工具类 UserUtils

/**
 *使用ThreadLocal存储共享的数据变量，如登录的用户信息
 */
public class UserUtils {
    private static  final  ThreadLocal<String> userLocal=new ThreadLocal<>();
 
    public static  String getUserId(){
        return userLocal.get();
    }
    public static void setUserId(String userId){
        userLocal.set(userId);
    }
 
    public static void clear(){
        userLocal.remove();
    }
 
}

自定义CustomTaskDecorator

/**
 * 线程池修饰类
 */
public class CustomTaskDecorator implements TaskDecorator {
    @Override
    public Runnable decorate(Runnable runnable) {
        // 获取主线程中的请求信息（我们的用户信息也放在里面）
        String robotId = UserUtils.getUserId();
        System.out.println(robotId);
        return () -> {
            try {
                // 将主线程的请求信息，设置到子线程中
                UserUtils.setUserId(robotId);
                // 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
                UserUtils.clear();
            }
        };
    }
}

ExecutorConfig

在原来的基础上增加 executor.setTaskDecorator(new CustomTaskDecorator());

@Bean(name = "asyncServiceExecutor")
    public Executor asyncServiceExecutor() {
        log.info("start asyncServiceExecutor----------------");
        //ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        //使用可视化运行状态的线程池
        ThreadPoolTaskExecutor executor = new VisiableThreadPoolTaskExecutor();
        //配置核心线程数
        executor.setCorePoolSize(corePoolSize);
        //配置最大线程数
        executor.setMaxPoolSize(maxPoolSize);
        //配置队列大小
        executor.setQueueCapacity(queueCapacity);
        //配置线程池中的线程的名称前缀
        executor.setThreadNamePrefix(namePrefix);
 
        // rejection-policy：当pool已经达到max size的时候，如何处理新任务
        // CALLER_RUNS：不在新线程中执行任务，而是有调用者所在的线程来执行
        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
 
        //增加线程池修饰类
        executor.setTaskDecorator(new CustomTaskDecorator());
        //增加MDC的线程池修饰类
        //executor.setTaskDecorator(new MDCTaskDecorator());
        //执行初始化
        executor.initialize();
        log.info("end asyncServiceExecutor------------");
        return executor;
    }

AsyncServiceImpl

    /**
     * 使用ThreadLocal方式传递
     * 带有返回值
     * @throws InterruptedException
     */
    @Async("asyncServiceExecutor")
    public CompletableFuture<String> executeValueAsync2() throws InterruptedException {
        log.info("start executeValueAsync");
        System.out.println("异步线程执行返回结果......+");
        log.info("end executeValueAsync");
        return CompletableFuture.completedFuture(UserUtils.getUserId());
    }
 

Test2Controller

    /**
     * 使用ThreadLocal+TaskDecorator的方式
     * @return
     * @throws InterruptedException
     * @throws ExecutionException
     */
    @GetMapping("/test2")
    public String test2() throws InterruptedException, ExecutionException {
        UserUtils.setUserId("123456");
        CompletableFuture<String> completableFuture = asyncService.executeValueAsync2();
        String s = completableFuture.get();
        return s;
    }

3 RequestContextHolder+TaskDecorator

自定义CustomTaskDecorator

/**
 * 线程池修饰类
 */
public class CustomTaskDecorator implements TaskDecorator {
    @Override
    public Runnable decorate(Runnable runnable) {
        // 获取主线程中的请求信息（我们的用户信息也放在里面）
        RequestAttributes attributes = RequestContextHolder.getRequestAttributes();
        return () -> {
            try {
                // 将主线程的请求信息，设置到子线程中
                RequestContextHolder.setRequestAttributes(attributes);
                // 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
                RequestContextHolder.resetRequestAttributes();
            }
        };
    }
}

ExecutorConfig

在原来的基础上增加 executor.setTaskDecorator(new CustomTaskDecorator());

@Bean(name = "asyncServiceExecutor")
    public Executor asyncServiceExecutor() {
        log.info("start asyncServiceExecutor----------------");
        //ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        //使用可视化运行状态的线程池
        ThreadPoolTaskExecutor executor = new VisiableThreadPoolTaskExecutor();
        //配置核心线程数
        executor.setCorePoolSize(corePoolSize);
        //配置最大线程数
        executor.setMaxPoolSize(maxPoolSize);
        //配置队列大小
        executor.setQueueCapacity(queueCapacity);
        //配置线程池中的线程的名称前缀
        executor.setThreadNamePrefix(namePrefix);
 
        // rejection-policy：当pool已经达到max size的时候，如何处理新任务
        // CALLER_RUNS：不在新线程中执行任务，而是有调用者所在的线程来执行
        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
 
        //增加线程池修饰类
        executor.setTaskDecorator(new CustomTaskDecorator());
        //增加MDC的线程池修饰类
        //executor.setTaskDecorator(new MDCTaskDecorator());
        //执行初始化
        executor.initialize();
        log.info("end asyncServiceExecutor------------");
        return executor;
    }

AsyncServiceImpl

     /**
     * 使用RequestAttributes获取主线程传递的数据
     * @return
     * @throws InterruptedException
     */
    @Async("asyncServiceExecutor")
    public CompletableFuture<String> executeValueAsync3() throws InterruptedException {
        log.info("start executeValueAsync");
        System.out.println("异步线程执行返回结果......+");
        RequestAttributes attributes = RequestContextHolder.getRequestAttributes();
        Object userId = attributes.getAttribute("userId", 0);
        log.info("end executeValueAsync");
        return CompletableFuture.completedFuture(userId.toString());
    }
 

Test2Controller

    /**
     * RequestContextHolder+TaskDecorator的方式
     * @return
     * @throws InterruptedException
     * @throws ExecutionException
     */
    @GetMapping("/test3")
    public String test3() throws InterruptedException, ExecutionException {
        RequestAttributes attributes = RequestContextHolder.getRequestAttributes();
        attributes.setAttribute("userId","123456",0);
        CompletableFuture<String> completableFuture = asyncService.executeValueAsync3();
        String s = completableFuture.get();
        return s;
    }

4 MDC+TaskDecorator

自定义MDCTaskDecorator

/**
 * 线程池修饰类
 */
public class MDCTaskDecorator implements TaskDecorator {
    @Override
    public Runnable decorate(Runnable runnable) {
        // 获取主线程中的请求信息（我们的用户信息也放在里面）
        String userId = MDC.get("userId");
        Map<String, String> copyOfContextMap = MDC.getCopyOfContextMap();
        System.out.println(copyOfContextMap);
        return () -> {
            try {
                // 将主线程的请求信息，设置到子线程中
                MDC.put("userId",userId);
                // 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
                MDC.clear();
            }
        };
    }
}

ExecutorConfig

在原来的基础上增加 executor.setTaskDecorator(new MDCTaskDecorator());

@Bean(name = "asyncServiceExecutor")
    public Executor asyncServiceExecutor() {
        log.info("start asyncServiceExecutor----------------");
        //ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        //使用可视化运行状态的线程池
        ThreadPoolTaskExecutor executor = new VisiableThreadPoolTaskExecutor();
        //配置核心线程数
        executor.setCorePoolSize(corePoolSize);
        //配置最大线程数
        executor.setMaxPoolSize(maxPoolSize);
        //配置队列大小
        executor.setQueueCapacity(queueCapacity);
        //配置线程池中的线程的名称前缀
        executor.setThreadNamePrefix(namePrefix);
 
        // rejection-policy：当pool已经达到max size的时候，如何处理新任务
        // CALLER_RUNS：不在新线程中执行任务，而是有调用者所在的线程来执行
        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
 
        //增加MDC的线程池修饰类
        executor.setTaskDecorator(new MDCTaskDecorator());
        //执行初始化
        executor.initialize();
        log.info("end asyncServiceExecutor------------");
        return executor;
    }

AsyncServiceImpl

         /**
     * 使用MDC获取主线程传递的数据
     * @return
     * @throws InterruptedException
     */
    @Async("asyncServiceExecutor")
    public CompletableFuture<String> executeValueAsync5() throws InterruptedException {
        log.info("start executeValueAsync");
        System.out.println("异步线程执行返回结果......+");
        log.info("end executeValueAsync");
        return CompletableFuture.completedFuture(MDC.get("userId"));
    }

Test2Controller

     /**
     * 使用MDC+TaskDecorator方式
     * 本质也是ThreadLocal+TaskDecorator方式
     * @return
     * @throws InterruptedException
     * @throws ExecutionException
     */
    @GetMapping("/test5")
    public String test5() throws InterruptedException, ExecutionException {
        MDC.put("userId","123456");
        CompletableFuture<String> completableFuture = asyncService.executeValueAsync5();
        String s = completableFuture.get();
        return s;
    }

5 InheritableThreadLocal

测试代码

public class TestThreadLocal {
	public static ThreadLocal<String> threadLocal = new ThreadLocal<>();
	public static void main(String[] args) {
		 //设置线程变量
        threadLocal.set("hello world");
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run( ) {
                //子线程输出线程变量的值
                System.out.println("thread:"+threadLocal.get());
            }
        });
        thread.start();
        // 主线程输出线程变量的值
        System.out.println("main:"+threadLocal.get());
	}
}

输出结果：

main:hello world
thread:null

从上面结果可以看出：同一个ThreadLocal变量在父线程中被设置后，在子线程中是获取不到的；

原因在子线程thread里面调用get方法时当前线程为thread线程，而这里调用set方法设置线程变量的是main线程，两者是不同的线程，自然子线程访问时返回null

为了解决上面的问题，InheritableThreadLocal应运而生，InheritableThreadLocal继承ThreadLocal，其提供一个特性，就是让子线程可以访问在父线程中设置的本地变量

将上面测试代码用InheritableThreadLocal修改

public class TestInheritableThreadLocal {
	
	public static InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
	
	public static void main(String[] args) {
		 //设置线程变量
        threadLocal.set("hello world");
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run( ) {
                //子线程输出线程变量的值
                System.out.println("thread:"+threadLocal.get());
            }
        });
        thread.start();
        // 主线程输出线程变量的值
        System.out.println("main:"+threadLocal.get());
	}
}

输出结果：

main:hello world
thread:hello world

5.1 源码分析

public class InheritableThreadLocal<T> extends ThreadLocal<T> {
    protected T childValue(T parentValue) {
        return parentValue;
    }
    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}

InheritableThreadLocal 重写了childValue，getMap，createMap三个方法
在InheritableThreadLocal中，变量inheritableThreadLocals 替代了threadLocals;

那么如何让子线程可以访问父线程的本地变量。这要从创建Thread的代码说起，打开Thread类的默认构造方法，代码如下：

  public Thread(Runnable target) {
        init(null, target, "Thread-" + nextThreadNum(), 0);
    }
 private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }
        this.name = name;
        //获取当前线程
        Thread parent = currentThread();
       //如果父线程的 inheritableThreadLocals变量不为null
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        //设置子线程inheritThreadLocals变量
            this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;
        /* Set thread ID */
        tid = nextThreadID();
    }

我们看下createInheritedMap代码：

this.inheritableThreadLocals =            ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

在createInheritedMap内部使用父线程的inheritableThreadLocals变量作为构造方法创建了一个新的ThreadLocalMap变量，然后赋值给子线程的inheritableThreadLocals变量。下面看看ThreadLocalMap的构造函数内部做了什么事情；

private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];
            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
                    if (key != null) {
                        Object value = key.childValue(e.value);
                        Entry c = new Entry(key, value);
                        int h = key.threadLocalHashCode & (len - 1);
                        while (table[h] != null)
                            h = nextIndex(h, len);
                        table[h] = c;
                        size++;
                    }
                }
            }
        }

InheritableThreadLocal 类通过重写下面代码

 ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }
    /**
     * Create the map associated with a ThreadLocal.
     *
     * @param t the current thread
     * @param firstValue value for the initial entry of the table.
     */
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }

让本地变量保存到了具体的线程的inheritableThreadLocals变量里面，那么线程在通过InheritableThreadLocal类实例的set或者get方法设置变量时，就会创建当前线程的inheritableThreadLocals变量。

当父线程创建子线程时，构造方法会把父线程中的inheritableThreadLocals变量里面的本地变量赋值一份保存到子线程的inheritableThreadLocals变量里面

5.2 InheritableThreadLocal存在的问题

虽然InheritableThreadLocal可以解决在子线程中获取父线程的值的问题，但是在使用线程池的情况下，由于不同的任务有可能是同一个线程处理，因此这些任务取到的值有可能并不是父线程设置的值
测试目标：任务1和任务2 获取父线程值一样，为测试代码中的hello world
测试代码：

public class TestInheritableThreadLocaIssue {
	
	public static InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
	public static ExecutorService executorService = Executors.newSingleThreadExecutor();
	
	public static void main(String[] args) throws Exception {
		 //设置线程变量
        threadLocal.set("hello world");
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run( ) {
                //子线程输出线程变量的值
                System.out.println("thread:"+threadLocal.get());
                threadLocal.set("hello world 2");
            }
        },"task1");
        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run( ) {
                //子线程输出线程变量的值
                System.out.println("thread:"+threadLocal.get());
                threadLocal.set("hello world 2");
            }
        },"task2");
        executorService.submit(thread1).get();
        executorService.submit(thread2).get();
        
        // 主线程输出线程变量的值
        System.out.println("main:"+threadLocal.get());
	}
}

输出结果：

thread:hello world
thread:hello world 2
main:hello world

结果分析：
很明显，任务2获取的不是父线程设置的hello world ，而是线程1修改后的值。如果在线程池中使用，需要注意这种情况（可以备份备份父线程的值）

6 TransmittableThreadLocal

解决线程池化值传递

阿里封装了一个工具，实现了在使用线程池等会池化复用线程的组件情况下，提供ThreadLocal值的传递功能，解决异步执行时上下文传递的问题

JDK的InheritableThreadLocal类可以完成父线程到子线程的值传递。但对于使用线程池等会池化复用线程的执行组件的情况，线程由线程池创建好，并且线程是池化起来反复使用的；这时父子线程关系的ThreadLocal值传递已经没有意义，应用需要的实际上是把 任务提交给线程池时的ThreadLocal值传递到 任务执行时
https://github.com/alibaba/transmittable-thread-local
引入：

<dependency>
	<groupId>com.alibaba</groupId>
	<artifactId>transmittable-thread-local</artifactId>
	<version>2.11.5</version>
</dependency>

需求场景：
1.分布式跟踪系统 或 全链路压测（即链路打标）
2.日志收集记录系统上下文
3.Session级Cache
4.应用容器或上层框架跨应用代码给下层SDK传递信息

测试代码：
1）父子线程信息传递

public static TransmittableThreadLocal<String> threadLocal = new TransmittableThreadLocal<>();
	
	public static void main(String[] args) {
		 //设置线程变量
        threadLocal.set("hello world");
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run( ) {
                //子线程输出线程变量的值
                System.out.println("thread:"+threadLocal.get());
            }
        });
        thread.start();
        // 主线程输出线程变量的值
        System.out.println("main:"+threadLocal.get());
	}
}

输出结果：

main:hello world
thread:hello world

2）线程池中传递值，参考github：修饰线程池