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线程安全反思录（上）：ThreadLocal到底安全不？

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背景

之前我有写过关于ThreadLocal的源码解析相关文章：一文带你读懂JDK源码：ThreadLocal类，但其实它不是业务最优解决方案，比如业界有更加先进的TransmittableThreadLocal可供选择。

下面我们按脑图来一起琢磨一下

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对ThreadLocal的思考

线程封闭：不同的Thread会指向不同的ThreadLocalMap对象，从而实现了线程封闭；而这个Map的key是ThreadLocal对象，value是需要缓存的值。

思考一：每个线程都会创建一个ThreadLocalMap，里面是一个Entry数组,会不会浪费内存空间吗？

答案是不会。

1.1 内存结构

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虽然每个Thread对象都有一个ThreadLocalMap，但这种设计并不是为了每个线程都创建一个全新的Map实例，而是为了实现线程局部变量的存储和访问。

每个ThreadLocal对象可以被多个线程共享，但在每个线程Thread内部，它们有自己的独立副本，即ThreadLocalMap。

这种设计允许线程安全地访问和修改数据，而不需要使用同步机制，从而提高了并发性能。

1.2 数据拓展性

同时ThreadLocalMap使用数组结构，一个Thread保存了一个ThreadLocalMap，它本质是一个Entry数组，恰好说明了，一个Thread是可以保存多个ThreadLocal对象值的。

key则是ThreadLocal.threadLocalHashCode 和 数组长度 进行位与运算的结果，恰好说明了，一个Thread是可以保存多个ThreadLocal对象值的。

这很好理解，比如在web后台系统里面，既有保存当前登录用户信息的ThreadLocal，也有保存当前请求trace_id链路标识的ThreadLocal，可能还有保存了租户id的ThreadLocal，它们都是不同的业务数据。

最终就是，一个Thread的一个Map可以存多个ThreadLocal数据，保存多个类型的数据。

这很好理解，比如在web后台系统里面，既有保存当前登录用户信息的ThreadLocal，也有保存当前请求trace_id链路标识的ThreadLocal，可能还有保存了租户id的ThreadLocal，它们都是不同的业务数据。

最终就是，一个Thread的一个Map可以存多个ThreadLocal数据。

思考二：ThreadLocal无法在父线程和子线程之间透传数据，有更好办法吗？

答案是有的。

修改方法也很简单：通过创建 InheritableThreadLocal 替代 ThreadLocal。

将

private static ThreadLocal<String> threadLocal = new ThreadLocal<>();

改为：

private static InheritableThreadLocal inheritableThreadLocal = new InheritableThreadLocal();

2.1 测试用例

@Slf4j
public class TestInheritableThreadLocal {

    private static InheritableThreadLocal inheritableThreadLocal = new InheritableThreadLocal();

    public static void main(String[] args) {
        inheritableThreadLocal.set("main");
        log.info("main - threadLocal:{}", inheritableThreadLocal.get());

        new Thread(() -> {
            log.info("main - thread - threadLocal:{}", inheritableThreadLocal.get());
        }).start();

        ThreadPoolExecutor threadPoolExecutor = ThreadPoolUtils.newThreadPool(
                1, 1,
                0, TimeUnit.SECONDS,
                new LinkedBlockingQueue<>(10), null, null);

        threadPoolExecutor.execute(new Runnable() {
            @Override
            public void run() {
                // 线程池里面的线程
                log.info("threadPoolExecutor - task1 - threadLocal:{}", inheritableThreadLocal.get());

                // 线程池里面的线程再new线程
                new Thread(() -> {
                    log.info("threadPoolExecutor - task1 - new thread - threadLocal:{}", inheritableThreadLocal.get());
                }).start();

            }
        });

        inheritableThreadLocal.set("main2");
        log.info("main - threadLocal:{}", inheritableThreadLocal.get());

        threadPoolExecutor.execute(new Runnable() {
            @Override
            public void run() {
                // 线程池里面的线程
                log.info("threadPoolExecutor - task2 - threadLocal:{}", inheritableThreadLocal.get());

                // 线程池里面的线程再new线程
                new Thread(() -> {
                    log.info("threadPoolExecutor - task2 - new thread - threadLocal:{}", inheritableThreadLocal.get());
                }).start();

            }
        });

    }
}
日志
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com.bryant.ttl.TestInheritableThreadLocal
21:04:39.203 [main] INFO com.bryant.ttl.TestInheritableThreadLocal - main - threadLocal:main
21:04:39.204 [Thread-0] INFO com.bryant.ttl.TestInheritableThreadLocal - main - thread - threadLocal:main
21:04:39.207 [main] INFO com.bryant.ttl.TestInheritableThreadLocal - main - threadLocal:main2
21:04:39.207 [pool-1-thread-1] INFO com.bryant.ttl.TestInheritableThreadLocal - threadPoolExecutor - task1 - threadLocal:main
21:04:39.207 [Thread-1] INFO com.bryant.ttl.TestInheritableThreadLocal - threadPoolExecutor - task1 - new thread - threadLocal:main

21:04:39.208 [pool-1-thread-1] INFO com.bryant.ttl.TestInheritableThreadLocal - threadPoolExecutor - task2 - threadLocal:main
21:04:39.208 [Thread-2] INFO com.bryant.ttl.TestInheritableThreadLocal - threadPoolExecutor - task2 - new thread - threadLocal:main

分析

  • 子线程可以获取父线程的上下文内容
  • 无论是通过ThreadPoolExecutor创建的线程，还是通过new Thread()创建的线程，都可以达到透传数据的目的
  • 但是，线程池执行两个异步线程里，主线程已经对InheritableThreadLocal进行了新的赋值（mian2字符串），但threadPoolExecutor第二次执行任务时，没有准确读入最新的InheritableThreadLocal，而是复用了旧线程的赋值（main字符串）

2.2 源码：InheritableThreadLocal

InheritableThreadLocal#set
public void set(T value) {
    Thread t = Thread.currentThread();
    // 【1】
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        map.set(this, value);
    } else {
        // 【2】
        createMap(t, value);
    }
}

ThreadLocalMap getMap(Thread t) {
   return t.inheritableThreadLocals;
}

void createMap(Thread t, T firstValue) {
    t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}

代码分析：

  • 【1】getMap(t)：如果线程T有inheritableThreadLocals，则返回它
  • 【2】createMap(t, value)：如果线程T没有inheritableThreadLocals，new了一个ThreadLocalMap，并赋值线程T的inheritableThreadLocals

我们观察到Thread除了包含threadLocals，还另外定义了inheritableThreadLocals。

public class Thread implements Runnable {
    ThreadLocal.ThreadLocalMap threadLocals = null;

    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;    
}
InheritableThreadLocal#get
public T get() {
    Thread t = Thread.currentThread();
    // 【3】
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        // 【4】
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    // 【5】
    return setInitialValue();
}

ThreadLocalMap getMap(Thread t) {
   return t.inheritableThreadLocals;
}

private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        map.set(this, value);
    } else {
        createMap(t, value);
    }
    if (this instanceof TerminatingThreadLocal) {
        TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
    }
    return value;
}

代码分析：

  • 【3】获取线程T的inheritableThreadLocals
  • 【4】如果不为空，返回ThreadLocalMap的值
  • 【5】如果为空，初始化ThreadLocalMap，并赋值线程T的inheritableThreadLocals
Thread的构造器
private Thread(ThreadGroup g, Runnable target, String name,
               long stackSize, AccessControlContext acc,
               boolean inheritThreadLocals) {
    //....

    Thread parent = currentThread();

    // 此处进行inheritThreadLocals的拷贝复制
    if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        this.inheritableThreadLocals =
            ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
    //....
}

代码分析：

  • 在新建线程时，会拷贝父线程的inheritableThreadLocals，到子线程的inheritableThreadLocals

思考三：InheritableThreadLocal有透传数据的优点，是否有坑？

答案是有的。

比如我们的MVC框架，默认是创建了一个名为“applicationTaskExecutor”线程池，所以一定存在线程复用的场景。

那么问题就来了。

如果请求1来了，上下文（用户名=A）放入了InheritableThreadLocal，然后处理掉请求1；

但由于线程复用，请求1结束处理之后，业务侧忘记清理上下文内存了，或者由于bug出现，漏设置新的上下文了；

“applicationTaskExecutor”线程池会用这个子线程继续处理请求2，此时的真实上下文是（用户名=B），但子线程拿到的还是（用户名=A）；

这也就是说，出现了内存泄漏。

解决办法也很多，在业务侧进行设计

  • 设置拦截器的preHandler，在一个请求处理前，对InheritableThreadLocal的数据赋值，设置最新的值
  • 设置拦截器的afterHandler，在一个请求处理完后，对InheritableThreadLocal的数据进行remove

但这个方案依旧有漏洞，因为业务内部也有线程池，这时候拦截器就无法对内部的线程进行拦截了，所以我们有更好的工具选择：阿里开源的TransmittableThreadLocal。

思考四：TransmittableThreadLocal的原理是什么？

3e33a5ea3858df7e83a1cadd89c59e33.png

4.1 测试用例

@Slf4j
public class TestTransmittableThreadLocal {

    private static TransmittableThreadLocal transmittableThreadLocal = new TransmittableThreadLocal();

    public static void main(String[] args) {
        transmittableThreadLocal.set("main");
        log.info("main - threadLocal:{}", transmittableThreadLocal.get());

        new Thread(() -> {
            log.info("thread1 - threadLocal:{}", transmittableThreadLocal.get());
            transmittableThreadLocal.set("thread-1");
            log.info("thread1 - after set - threadLocal:{}", transmittableThreadLocal.get());
        }).start();

        Executor threadPoolExecutor = TtlExecutors.getTtlExecutor(
                ThreadPoolUtils.newThreadPool(
                1, 1,
                0, TimeUnit.SECONDS,
                new LinkedBlockingQueue<>(10), null, null));

        // 构造一个新的TtlRunnable
        TtlRunnable ttlRunnable = TtlRunnable.get(new Runnable() {
            @Override
            public void run() {
                // 线程池里面的线程
                log.info("threadPoolExecutor - task - threadLocal:{}", transmittableThreadLocal.get());

                // 线程池里面的线程再new线程
                new Thread(() -> {
                    log.info("threadPoolExecutor - task - thread - threadLocal:{}", transmittableThreadLocal.get());
                }).start();

            }
        });
        threadPoolExecutor.execute(ttlRunnable);

        transmittableThreadLocal.set("main2");
        log.info("main - threadLocal:{}", transmittableThreadLocal.get());

        // 构造一个新的TtlRunnable
        TtlRunnable ttlRunnable2 =TtlRunnable.get(new Runnable() {
            @Override
            public void run() {
                // 线程池里面的线程
                log.info("threadPoolExecutor - task2 - threadLocal:{}", transmittableThreadLocal.get());

                // 线程池里面的线程再new线程
                new Thread(() -> {
                    log.info("threadPoolExecutor - task2 - new thread - threadLocal:{}", transmittableThreadLocal.get());
                }).start();

            }
        });
        threadPoolExecutor.execute(ttlRunnable2);

    }
}
日志
22:01:38.358 [main] INFO com.bryant.ttl.TestTransmittableThreadLocal - main - threadLocal:main
22:01:38.360 [Thread-0] INFO com.bryant.ttl.TestTransmittableThreadLocal - thread1 - threadLocal:main
22:01:38.361 [Thread-0] INFO com.bryant.ttl.TestTransmittableThreadLocal - thread1 - after set - threadLocal:thread-1
22:01:38.363 [main] INFO com.bryant.ttl.TestTransmittableThreadLocal - main - threadLocal:main2
22:01:38.364 [pool-1-thread-1] INFO com.bryant.ttl.TestTransmittableThreadLocal - threadPoolExecutor - task - threadLocal:main
22:01:38.364 [pool-1-thread-1] INFO com.bryant.ttl.TestTransmittableThreadLocal - threadPoolExecutor - task2 - threadLocal:main2
22:01:38.364 [Thread-1] INFO com.bryant.ttl.TestTransmittableThreadLocal - threadPoolExecutor - task - thread - threadLocal:main
22:01:38.365 [Thread-2] INFO com.bryant.ttl.TestTransmittableThreadLocal - threadPoolExecutor - task2 - new thread - threadLocal:main2
分析
  • 子线程可以获取父线程的上下文内容
  • 无论是通过ThreadPoolExecutor创建的线程，还是通过new Thread()创建的线程，都可以达到透传数据的目的
  • 对于main线程做的修改，即使是通过线程复用，也能够实时透传给子线程

4.2 源码：分析TransmittableThreadLocal

private static TransmittableThreadLocal transmittableThreadLocal = new TransmittableThreadLocal();
TransmittableThreadLocal#set
private static final InheritableThreadLocal<WeakHashMap<TransmittableThreadLocal<Object>, ?>> holder =
        new InheritableThreadLocal<WeakHashMap<TransmittableThreadLocal<Object>, ?>>() {
            @Override
            protected WeakHashMap<TransmittableThreadLocal<Object>, ?> 
                initialValue() {
                return new WeakHashMap<TransmittableThreadLocal<Object>, Object>();
            }

            @Override
            protected WeakHashMap<TransmittableThreadLocal<Object>, ?> 
                childValue(WeakHashMap<TransmittableThreadLocal<Object>, ?> parentValue) {
                return new WeakHashMap<TransmittableThreadLocal<Object>, Object>(parentValue);
            }
        };

@Override
public final void set(T value) {
    if (!disableIgnoreNullValueSemantics && null == value) {
        // 【1】
        remove();
    } else {
         // 【2】
        super.set(value);
         // 【3】
        addThisToHolder();
    }
}

@Override
public final void remove() {
    removeThisFromHolder();
    super.remove();
}

private void removeThisFromHolder() {
    holder.get().remove(this);
}

// java.lang.ThreadLocal#remove
public void remove() {
    ThreadLocalMap m = getMap(Thread.currentThread());
    if (m != null) {
        m.remove(this);
    }
}

代码分析：

  • 【1】若 disableIgnoreNullValueSemantics 为 false 且传入值 value 为 null，则调用 remove() 方法移除值。
  • 【2】否则，调用父类的 set 方法设置值，即，java.lang.ThreadLocal#set
  • 【3】并通过 addThisToHolder 将当前对象添加TransmittableThreadLocal的holder中
TransmittableThreadLocal#get
@Override
public final T get() {
    T value = super.get();
    if (disableIgnoreNullValueSemantics || null != value) addThisToHolder();
    return value;
}

private void addThisToHolder() {
    if (!holder.get().containsKey(this)) {
        holder.get().put((TransmittableThreadLocal<Object>) this, null); // WeakHashMap supports null value.
    }
}

代码分析：

  • 从父类获取缓存值 value。
  • 检查是否禁用忽略空值语义或 value 非空，若是，则将当前对象添加到持有者中。
  • 返回获取的 value。
TtlExecutors#getTtlExecutor
public final class TtlExecutors {
    @Nullable
    public static Executor getTtlExecutor(@Nullable Executor executor) {
        // 【1】
        if (TtlAgent.isTtlAgentLoaded() || null == executor || executor instanceof TtlEnhanced) {
            return executor;
        }
        // 【2】
        return new ExecutorTtlWrapper(executor, true);
    }
}

class ExecutorTtlWrapper implements Executor, TtlWrapper<Executor>, TtlEnhanced {
    private final Executor executor;
    protected final boolean idempotent;

    //【3】
    ExecutorTtlWrapper(@NonNull Executor executor, boolean idempotent) {
        this.executor = executor;
        this.idempotent = idempotent;
    }

}

代码分析：

  • 【1】若Ttl代理已加载或传入执行器为null或已是Ttl增强版，则直接返回传入的执行器
  • 【2】否则，返回一个包装后的ExecutorTtlWrapper实例。
  • 【3】构造器会把真正执行任务的线程池，封装起来，而ExecutorTtlWrapper作为一个代理类被调用。
TtlRunnable#get
public static TtlRunnable get(@Nullable Runnable runnable, boolean releaseTtlValueReferenceAfterRun, boolean idempotent) {
    if (null == runnable) return null;

    if (runnable instanceof TtlEnhanced) {
        // avoid redundant decoration, and ensure idempotency
        if (idempotent) return (TtlRunnable) runnable;
        else throw new IllegalStateException("Already TtlRunnable!");
    }
    return new TtlRunnable(runnable, releaseTtlValueReferenceAfterRun);
}

代码分析：

根据给定的参数获取一个TtlRunnable实例：

  • 如果传入的runnable为null，返回null。
  • 如果runnable是TtlEnhanced的实例且idempotent为true，直接返回runnable作为TtlRunnable。
  • 否则，创建一个新的TtlRunnable实例并返回。如果runnable已是TtlRunnable实例且idempotent为false，抛出异常。
核心：创建一个新的TtlRunnable实例
Snapshot快照生成
private final AtomicReference<Object> capturedRef;

public final class TtlRunnable implements Runnable, TtlWrapper<Runnable>, TtlEnhanced, TtlAttachments {
    private TtlRunnable(@NonNull Runnable runnable, boolean releaseTtlValueReferenceAfterRun) {
        //【1】
        this.capturedRef = new AtomicReference<Object>(capture());
        //【2】
        this.runnable = runnable;
        this.releaseTtlValueReferenceAfterRun = releaseTtlValueReferenceAfterRun;
    }
}

public static class Transmitter {
    @NonNull
    public static Object capture() {
        // 【1.1】
        return new Snapshot(captureTtlValues(), captureThreadLocalValues());
    }

    private static HashMap<TransmittableThreadLocal<Object>, Object> captureTtlValues() {
        HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value = new HashMap<TransmittableThreadLocal<Object>, Object>();
        for (TransmittableThreadLocal<Object> threadLocal : holder.get().keySet()) {
            // 【1.2】
            ttl2Value.put(threadLocal, threadLocal.copyValue());
        }
        return ttl2Value;
    }

    private static HashMap<ThreadLocal<Object>, Object> captureThreadLocalValues() {
        final HashMap<ThreadLocal<Object>, Object> threadLocal2Value = new HashMap<ThreadLocal<Object>, Object>();
        for (Map.Entry<ThreadLocal<Object>, TtlCopier<Object>> entry : threadLocalHolder.entrySet()) {
            final ThreadLocal<Object> threadLocal = entry.getKey();
            final TtlCopier<Object> copier = entry.getValue();
            // 【1.3】
            threadLocal2Value.put(threadLocal, copier.copy(threadLocal.get()));
        }
        return threadLocal2Value;
    }
}

private static class Snapshot {
    final HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value;
    final HashMap<ThreadLocal<Object>, Object> threadLocal2Value;

    private Snapshot(HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value, HashMap<ThreadLocal<Object>, Object> threadLocal2Value) {
        this.ttl2Value = ttl2Value;
        this.threadLocal2Value = threadLocal2Value;
    }
}

代码分析：

  • 【1】capturedRef方法取到快照对象Snapshot，赋值给AtomicReference原子类- captureTtlValues()：Snapshot的ttl2Value，数据来源于holder的注册值- captureThreadLocalValues()：Snapshot的threadLocal2Value，数据来源于threadLocalHolder的注册值- capture()：构造了Snapshot对象
  • 【2】任务缓存起来

通过上面的源码分析，我们可以得到下面的流程图：

只要异步线程执行前，调用了TransmittableThreadLocal#set，那么就会触发addThisToHolder对holder的TransmittableThreadLocal进行数据更新

24e0ec050450ae701ee2a0d632bad4df.png

Snapshot快照重放
public final class TtlRunnable implements Runnable, TtlWrapper<Runnable>, TtlEnhanced, TtlAttachments {
    @Override
    public void run() {
         // 【1】
        final Object captured = capturedRef.get();
        if (captured == null || releaseTtlValueReferenceAfterRun && !capturedRef.compareAndSet(captured, null)) {
            throw new IllegalStateException("TTL value reference is released after run!");
        }
        //【2】
        final Object backup = replay(captured);
        try {
            // 【3】
            runnable.run();
        } finally {
            // 【4】
            restore(backup);
        }
    }

    @NonNull
    public static Object replay(@NonNull Object captured) {
        final Snapshot capturedSnapshot = (Snapshot) captured;
        return new Snapshot(replayTtlValues(capturedSnapshot.ttl2Value), replayThreadLocalValues(capturedSnapshot.threadLocal2Value));
    }

    public static void restore(@NonNull Object backup) {
        final Snapshot backupSnapshot = (Snapshot) backup;
        restoreTtlValues(backupSnapshot.ttl2Value);
        restoreThreadLocalValues(backupSnapshot.threadLocal2Value);
    }
}

代码分析：

  • 线程池执行TtlRunnable任务时：- holder值恢复- threadLocalHolder值恢复- 【1】任务执行前 - 获取快照Snapshot1- 【2】任务执行前 - 重新构造一个新快照对象Snapshot2- 【3】任务执行 runnable.run();- 【4】任务执行后 - 重放Snapshot2

通过上面的源码分析，我们可以得到下面的流程图：

确保了线程复用时，能够获取到线程执行前最新的数据，而不用担心上一次执行后留下的脏数据结果

044b77154f1df1ae17e3d4d490cdf9cd.png

ExecutorTtlWrapper#execute：封装好的TTL线程池执行任务

class ExecutorTtlWrapper implements Executor, TtlWrapper<Executor>, TtlEnhanced {
    private final Executor executor;

    @Override
    public void execute(@NonNull Runnable command) {
        executor.execute(TtlRunnable.get(command, false, idempotent));
    }
}

代码分析：

  • TtlRunnable.get：参考上面的 TtlRunnable#get
  • executor.execute：封装好的线程池，执行任务即可

思考五：三种工具对比

优点

缺点

ThreadLocal

简单使用，每个线程都有独立的变量副本，避免了线程安全问题。

不能跨线程传递变量值，不适用于需要在线程间共享数据的场景。

InheritableThreadLocal

允许父子线程间传递变量值，适用于需要传递上下文信息的场景。

不适用于线程池的线程复用场景，因为线程池中的线程可能不是父子关系。

另外，InheritableThreadLocal的传递是发生在新建线程的时候，但是在我们项目中基本都是用线程池来使用多线程，因为线程的创建销毁都会有资源消耗，当然也有可能发生oom

TransmittableThreadLocal

支持在线程池等场景中传递变量值，适用于需要跨多个线程共享数据的场景

相比ThreadLocal和InheritableThreadLocal，实现更复杂，使用时需要额外注意线程安全

拓展：SpringMVC异步线程池去接每个请求的，可能出现的内存泄漏？

因为SpringBoot应用，会通过@SpringBootApplication来自动加载相关系统配置类，所以我们了解一下加载类，先看spring.factories：

297dc70ae7f984a5497b7323a53a626a.png

里面有一个类：

org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration

TaskExecutionAutoConfiguration

@ConditionalOnClass(ThreadPoolTaskExecutor.class)
@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(TaskExecutionProperties.class)
public class TaskExecutionAutoConfiguration {

   /**
    * Bean name of the application {@link TaskExecutor}.
    */
   public static final String APPLICATION_TASK_EXECUTOR_BEAN_NAME = "applicationTaskExecutor";

   @Bean
   @ConditionalOnMissingBean
   public TaskExecutorBuilder taskExecutorBuilder(TaskExecutionProperties properties,
         ObjectProvider<TaskExecutorCustomizer> taskExecutorCustomizers,
         ObjectProvider<TaskDecorator> taskDecorator) {
      TaskExecutionProperties.Pool pool = properties.getPool();
      TaskExecutorBuilder builder = new TaskExecutorBuilder();
      builder = builder.queueCapacity(pool.getQueueCapacity());
      builder = builder.corePoolSize(pool.getCoreSize());
      builder = builder.maxPoolSize(pool.getMaxSize());
      builder = builder.allowCoreThreadTimeOut(pool.isAllowCoreThreadTimeout());
      builder = builder.keepAlive(pool.getKeepAlive());
      Shutdown shutdown = properties.getShutdown();
      builder = builder.awaitTermination(shutdown.isAwaitTermination());
      builder = builder.awaitTerminationPeriod(shutdown.getAwaitTerminationPeriod());
      builder = builder.threadNamePrefix(properties.getThreadNamePrefix());
      builder = builder.customizers(taskExecutorCustomizers.orderedStream()::iterator);
      builder = builder.taskDecorator(taskDecorator.getIfUnique());
      return builder;
   }

   @Lazy
   @Bean(name = { APPLICATION_TASK_EXECUTOR_BEAN_NAME,
         AsyncAnnotationBeanPostProcessor.DEFAULT_TASK_EXECUTOR_BEAN_NAME })
   @ConditionalOnMissingBean(Executor.class)
   public ThreadPoolTaskExecutor applicationTaskExecutor(TaskExecutorBuilder builder) {
      return builder.build();
   }

}

代码分析：

  • 这个配置类，强制要求有ThreadPoolTaskExecutor才可以初始化
  • TaskExecutionProperties：spring.task.execution开头的配置项，我们可以基于此进行定制化
  • 通过建造者模式，使用TaskExecutorBuilder进行的ThreadPoolTaskExecutor封装构建。

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