线程安全反思录（上）：ThreadLocal到底安全不？

  

  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办理方案，比如业界有更加先辈的TransmittableThreadLocal可供选择。
  下面我们按脑图来一起琢磨一下
  

  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

  

  虽然每个Thread对象都有一个ThreadLocalMap，但这种计划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。
  这种计划允许线程安全地访问和修改数据，而不需要使用同步机制，从而进步了并发性能。
  1.2 数据拓展性

  同时ThreadLocalMap使用数组结构，一个Thread生存了一个ThreadLocalMap，它本质是一个Entry数组，恰好阐明了，一个Thread是可以生存多个ThreadLocal对象值的。
  key则是ThreadLocal.threadLocalHashCode 和 数组长度 举行位与运算的效果，恰好阐明了，一个Thread是可以生存多个ThreadLocal对象值的。
  这很好理解，比如在web背景系统里面，既有生存当前登任命户信息的ThreadLocal，也有生存当前请求trace_id链路标识的ThreadLocal，可能另有生存了租户id的ThreadLocal，它们都是不同的业务数据。
  终极就是，一个Thread的一个Map可以存多个ThreadLocal数据，生存多个范例的数据。
  这很好理解，比如在web背景系统里面，既有生存当前登任命户信息的ThreadLocal，也有生存当前请求trace_id链路标识的ThreadLocal，可能另有生存了租户id的ThreadLocal，它们都是不同的业务数据。
  终极就是，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

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

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改为：

1. private static InheritableThreadLocal inheritableThreadLocal = new InheritableThreadLocal();

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2.1 测试用例

1. @Slf4j
2. public class TestInheritableThreadLocal {
5.     private static InheritableThreadLocal inheritableThreadLocal = new InheritableThreadLocal();
8.     public static void main(String[] args) {
9.         inheritableThreadLocal.set("main");
10.         log.info("main - threadLocal:{}", inheritableThreadLocal.get());
13.         new Thread(() -> {
14.             log.info("main - thread - threadLocal:{}", inheritableThreadLocal.get());
15.         }).start();
18.         ThreadPoolExecutor threadPoolExecutor = ThreadPoolUtils.newThreadPool(
19.                 1, 1,
20.                 0, TimeUnit.SECONDS,
21.                 new LinkedBlockingQueue<>(10), null, null);
24.         threadPoolExecutor.execute(new Runnable() {
25.             @Override
26.             public void run() {
27.                 // 线程池里面的线程
28.                 log.info("threadPoolExecutor - task1 - threadLocal:{}", inheritableThreadLocal.get());
31.                 // 线程池里面的线程再new线程
32.                 new Thread(() -> {
33.                     log.info("threadPoolExecutor - task1 - new thread - threadLocal:{}", inheritableThreadLocal.get());
34.                 }).start();
37.             }
38.         });
41.         inheritableThreadLocal.set("main2");
42.         log.info("main - threadLocal:{}", inheritableThreadLocal.get());
45.         threadPoolExecutor.execute(new Runnable() {
46.             @Override
47.             public void run() {
48.                 // 线程池里面的线程
49.                 log.info("threadPoolExecutor - task2 - threadLocal:{}", inheritableThreadLocal.get());
52.                 // 线程池里面的线程再new线程
53.                 new Thread(() -> {
54.                     log.info("threadPoolExecutor - task2 - new thread - threadLocal:{}", inheritableThreadLocal.get());
55.                 }).start();
58.             }
59.         });
62.     }
63. }

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日记

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com.bryant.ttl.TestInheritableThreadLocal
2. 21:04:39.203 [main] INFO com.bryant.ttl.TestInheritableThreadLocal - main - threadLocal:main
3. 21:04:39.204 [Thread-0] INFO com.bryant.ttl.TestInheritableThreadLocal - main - thread - threadLocal:main
4. 21:04:39.207 [main] INFO com.bryant.ttl.TestInheritableThreadLocal - main - threadLocal:main2
5. 21:04:39.207 [pool-1-thread-1] INFO com.bryant.ttl.TestInheritableThreadLocal - threadPoolExecutor - task1 - threadLocal:main
6. 21:04:39.207 [Thread-1] INFO com.bryant.ttl.TestInheritableThreadLocal - threadPoolExecutor - task1 - new thread - threadLocal:main
9. 21:04:39.208 [pool-1-thread-1] INFO com.bryant.ttl.TestInheritableThreadLocal - threadPoolExecutor - task2 - threadLocal:main
10. 21:04:39.208 [Thread-2] INFO com.bryant.ttl.TestInheritableThreadLocal - threadPoolExecutor - task2 - new thread - threadLocal:main

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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标
  
• 但是，线程池执行两个异步线程里，主线程已经对InheritableThreadLocal举行了新的赋值（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

1. public void set(T value) {
2.     Thread t = Thread.currentThread();
3.     // 【1】
4.     ThreadLocalMap map = getMap(t);
5.     if (map != null) {
6.         map.set(this, value);
7.     } else {
8.         // 【2】
9.         createMap(t, value);
10.     }
11. }
16. ThreadLocalMap getMap(Thread t) {
17.    return t.inheritableThreadLocals;
18. }
21. void createMap(Thread t, T firstValue) {
22.     t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
23. }

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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，还别的界说了inheritableThreadLocals。

1. public class Thread implements Runnable {
2.     ThreadLocal.ThreadLocalMap threadLocals = null;
5.     ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;   
6. }

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InheritableThreadLocal#get

1. public T get() {
2.     Thread t = Thread.currentThread();
3.     // 【3】
4.     ThreadLocalMap map = getMap(t);
5.     if (map != null) {
6.         // 【4】
7.         ThreadLocalMap.Entry e = map.getEntry(this);
8.         if (e != null) {
9.             @SuppressWarnings("unchecked")
10.             T result = (T)e.value;
11.             return result;
12.         }
13.     }
14.     // 【5】
15.     return setInitialValue();
16. }
19. ThreadLocalMap getMap(Thread t) {
20.    return t.inheritableThreadLocals;
21. }
24. private T setInitialValue() {
25.     T value = initialValue();
26.     Thread t = Thread.currentThread();
27.     ThreadLocalMap map = getMap(t);
28.     if (map != null) {
29.         map.set(this, value);
30.     } else {
31.         createMap(t, value);
32.     }
33.     if (this instanceof TerminatingThreadLocal) {
34.         TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
35.     }
36.     return value;
37. }

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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

1. private Thread(ThreadGroup g, Runnable target, String name,
2.                long stackSize, AccessControlContext acc,
3.                boolean inheritThreadLocals) {
4.     //....
7.     Thread parent = currentThread();
10.     // 此处进行inheritThreadLocals的拷贝复制
11.     if (inheritThreadLocals && parent.inheritableThreadLocals != null)
12.         this.inheritableThreadLocals =
13.             ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
14.     //....
15. }

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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有透传数据的长处，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。
  办理办法也许多，在业务侧举行计划
  

• 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的数据举行remove
  

  但这个方案仍旧有漏洞，由于业务内部也有线程池，这时候拦截器就无法对内部的线程举行拦截了，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？

  

  4.1 测试用例

1. @Slf4j
2. public class TestTransmittableThreadLocal {
5.     private static TransmittableThreadLocal transmittableThreadLocal = new TransmittableThreadLocal();
8.     public static void main(String[] args) {
9.         transmittableThreadLocal.set("main");
10.         log.info("main - threadLocal:{}", transmittableThreadLocal.get());
13.         new Thread(() -> {
14.             log.info("thread1 - threadLocal:{}", transmittableThreadLocal.get());
15.             transmittableThreadLocal.set("thread-1");
16.             log.info("thread1 - after set - threadLocal:{}", transmittableThreadLocal.get());
17.         }).start();
20.         Executor threadPoolExecutor = TtlExecutors.getTtlExecutor(
21.                 ThreadPoolUtils.newThreadPool(
22.                 1, 1,
23.                 0, TimeUnit.SECONDS,
24.                 new LinkedBlockingQueue<>(10), null, null));
27.         // 构造一个新的TtlRunnable
28.         TtlRunnable ttlRunnable = TtlRunnable.get(new Runnable() {
29.             @Override
30.             public void run() {
31.                 // 线程池里面的线程
32.                 log.info("threadPoolExecutor - task - threadLocal:{}", transmittableThreadLocal.get());
35.                 // 线程池里面的线程再new线程
36.                 new Thread(() -> {
37.                     log.info("threadPoolExecutor - task - thread - threadLocal:{}", transmittableThreadLocal.get());
38.                 }).start();
41.             }
42.         });
43.         threadPoolExecutor.execute(ttlRunnable);
46.         transmittableThreadLocal.set("main2");
47.         log.info("main - threadLocal:{}", transmittableThreadLocal.get());
50.         // 构造一个新的TtlRunnable
51.         TtlRunnable ttlRunnable2 =TtlRunnable.get(new Runnable() {
52.             @Override
53.             public void run() {
54.                 // 线程池里面的线程
55.                 log.info("threadPoolExecutor - task2 - threadLocal:{}", transmittableThreadLocal.get());
58.                 // 线程池里面的线程再new线程
59.                 new Thread(() -> {
60.                     log.info("threadPoolExecutor - task2 - new thread - threadLocal:{}", transmittableThreadLocal.get());
61.                 }).start();
64.             }
65.         });
66.         threadPoolExecutor.execute(ttlRunnable2);
69.     }
70. }

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日记

1. 22:01:38.358 [main] INFO com.bryant.ttl.TestTransmittableThreadLocal - main - threadLocal:main
2. 22:01:38.360 [Thread-0] INFO com.bryant.ttl.TestTransmittableThreadLocal - thread1 - threadLocal:main
3. 22:01:38.361 [Thread-0] INFO com.bryant.ttl.TestTransmittableThreadLocal - thread1 - after set - threadLocal:thread-1
4. 22:01:38.363 [main] INFO com.bryant.ttl.TestTransmittableThreadLocal - main - threadLocal:main2
5. 22:01:38.364 [pool-1-thread-1] INFO com.bryant.ttl.TestTransmittableThreadLocal - threadPoolExecutor - task - threadLocal:main
6. 22:01:38.364 [pool-1-thread-1] INFO com.bryant.ttl.TestTransmittableThreadLocal - threadPoolExecutor - task2 - threadLocal:main2
7. 22:01:38.364 [Thread-1] INFO com.bryant.ttl.TestTransmittableThreadLocal - threadPoolExecutor - task - thread - threadLocal:main
8. 22:01:38.365 [Thread-2] INFO com.bryant.ttl.TestTransmittableThreadLocal - threadPoolExecutor - task2 - new thread - threadLocal:main2

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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标
  
• 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

1. private static TransmittableThreadLocal transmittableThreadLocal = new TransmittableThreadLocal();

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TransmittableThreadLocal#set

1. private static final InheritableThreadLocal<WeakHashMap<TransmittableThreadLocal<Object>, ?>> holder =
2.         new InheritableThreadLocal<WeakHashMap<TransmittableThreadLocal<Object>, ?>>() {
3.             @Override
4.             protected WeakHashMap<TransmittableThreadLocal<Object>, ?>
5.                 initialValue() {
6.                 return new WeakHashMap<TransmittableThreadLocal<Object>, Object>();
7.             }
10.             @Override
11.             protected WeakHashMap<TransmittableThreadLocal<Object>, ?>
12.                 childValue(WeakHashMap<TransmittableThreadLocal<Object>, ?> parentValue) {
13.                 return new WeakHashMap<TransmittableThreadLocal<Object>, Object>(parentValue);
14.             }
15.         };
18. @Override
19. public final void set(T value) {
20.     if (!disableIgnoreNullValueSemantics && null == value) {
21.         // 【1】
22.         remove();
23.     } else {
24.          // 【2】
25.         super.set(value);
26.          // 【3】
27.         addThisToHolder();
28.     }
29. }
32. @Override
33. public final void remove() {
34.     removeThisFromHolder();
35.     super.remove();
36. }
39. private void removeThisFromHolder() {
40.     holder.get().remove(this);
41. }
44. // java.lang.ThreadLocal#remove
45. public void remove() {
46.     ThreadLocalMap m = getMap(Thread.currentThread());
47.     if (m != null) {
48.         m.remove(this);
49.     }
50. }

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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

1. @Override
2. public final T get() {
3.     T value = super.get();
4.     if (disableIgnoreNullValueSemantics || null != value) addThisToHolder();
5.     return value;
6. }
9. private void addThisToHolder() {
10.     if (!holder.get().containsKey(this)) {
11.         holder.get().put((TransmittableThreadLocal<Object>) this, null); // WeakHashMap supports null value.
12.     }
13. }

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代码分析：
  

• 从父类获取缓存值 value。
  
• 查抄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

1. public final class TtlExecutors {
2.     @Nullable
3.     public static Executor getTtlExecutor(@Nullable Executor executor) {
4.         // 【1】
5.         if (TtlAgent.isTtlAgentLoaded() || null == executor || executor instanceof TtlEnhanced) {
6.             return executor;
7.         }
8.         // 【2】
9.         return new ExecutorTtlWrapper(executor, true);
10.     }
11. }
14. class ExecutorTtlWrapper implements Executor, TtlWrapper<Executor>, TtlEnhanced {
15.     private final Executor executor;
16.     protected final boolean idempotent;
19.     //【3】
20.     ExecutorTtlWrapper(@NonNull Executor executor, boolean idempotent) {
21.         this.executor = executor;
22.         this.idempotent = idempotent;
23.     }
26. }

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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

1. public static TtlRunnable get(@Nullable Runnable runnable, boolean releaseTtlValueReferenceAfterRun, boolean idempotent) {
2.     if (null == runnable) return null;
5.     if (runnable instanceof TtlEnhanced) {
6.         // avoid redundant decoration, and ensure idempotency
7.         if (idempotent) return (TtlRunnable) runnable;
8.         else throw new IllegalStateException("Already TtlRunnable!");
9.     }
10.     return new TtlRunnable(runnable, releaseTtlValueReferenceAfterRun);
11. }

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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。
  

  焦点：创建一个新的TtlRunnable实例

  Snapshot快照生成

1. private final AtomicReference<Object> capturedRef;
4. public final class TtlRunnable implements Runnable, TtlWrapper<Runnable>, TtlEnhanced, TtlAttachments {
5.     private TtlRunnable(@NonNull Runnable runnable, boolean releaseTtlValueReferenceAfterRun) {
6.         //【1】
7.         this.capturedRef = new AtomicReference<Object>(capture());
8.         //【2】
9.         this.runnable = runnable;
10.         this.releaseTtlValueReferenceAfterRun = releaseTtlValueReferenceAfterRun;
11.     }
12. }
15. public static class Transmitter {
16.     @NonNull
17.     public static Object capture() {
18.         // 【1.1】
19.         return new Snapshot(captureTtlValues(), captureThreadLocalValues());
20.     }
23.     private static HashMap<TransmittableThreadLocal<Object>, Object> captureTtlValues() {
24.         HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value = new HashMap<TransmittableThreadLocal<Object>, Object>();
25.         for (TransmittableThreadLocal<Object> threadLocal : holder.get().keySet()) {
26.             // 【1.2】
27.             ttl2Value.put(threadLocal, threadLocal.copyValue());
28.         }
29.         return ttl2Value;
30.     }
33.     private static HashMap<ThreadLocal<Object>, Object> captureThreadLocalValues() {
34.         final HashMap<ThreadLocal<Object>, Object> threadLocal2Value = new HashMap<ThreadLocal<Object>, Object>();
35.         for (Map.Entry<ThreadLocal<Object>, TtlCopier<Object>> entry : threadLocalHolder.entrySet()) {
36.             final ThreadLocal<Object> threadLocal = entry.getKey();
37.             final TtlCopier<Object> copier = entry.getValue();
38.             // 【1.3】
39.             threadLocal2Value.put(threadLocal, copier.copy(threadLocal.get()));
40.         }
41.         return threadLocal2Value;
42.     }
43. }
46. private static class Snapshot {
47.     final HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value;
48.     final HashMap<ThreadLocal<Object>, Object> threadLocal2Value;
51.     private Snapshot(HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value, HashMap<ThreadLocal<Object>, Object> threadLocal2Value) {
52.         this.ttl2Value = ttl2Value;
53.         this.threadLocal2Value = threadLocal2Value;
54.     }
55. }

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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，数据泉源于holder的注册值
    
  • captureThreadLocalValues()：Snapshot的threadLocal2Value，数据泉源于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举行数据更新
  

  Snapshot快照重放

1. public final class TtlRunnable implements Runnable, TtlWrapper<Runnable>, TtlEnhanced, TtlAttachments {
2.     @Override
3.     public void run() {
4.          // 【1】
5.         final Object captured = capturedRef.get();
6.         if (captured == null || releaseTtlValueReferenceAfterRun && !capturedRef.compareAndSet(captured, null)) {
7.             throw new IllegalStateException("TTL value reference is released after run!");
8.         }
9.         //【2】
10.         final Object backup = replay(captured);
11.         try {
12.             // 【3】
13.             runnable.run();
14.         } finally {
15.             // 【4】
16.             restore(backup);
17.         }
18.     }
21.     @NonNull
22.     public static Object replay(@NonNull Object captured) {
23.         final Snapshot capturedSnapshot = (Snapshot) captured;
24.         return new Snapshot(replayTtlValues(capturedSnapshot.ttl2Value), replayThreadLocalValues(capturedSnapshot.threadLocal2Value));
25.     }
28.     public static void restore(@NonNull Object backup) {
29.         final Snapshot backupSnapshot = (Snapshot) backup;
30.         restoreTtlValues(backupSnapshot.ttl2Value);
31.         restoreThreadLocalValues(backupSnapshot.threadLocal2Value);
32.     }
33. }

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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效果
  

  

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

1. class ExecutorTtlWrapper implements Executor, TtlWrapper<Executor>, TtlEnhanced {
2.     private final Executor executor;
5.     @Override
6.     public void execute(@NonNull Runnable command) {
7.         executor.execute(TtlRunnable.get(command, false, idempotent));
8.     }
9. }

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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

  

	长处	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，由于线程池中的线程可能不是父子关系。 别的，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，由于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？

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

  里面有一个类：

1. org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration

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TaskExecutionAutoConfiguration

1. @ConditionalOnClass(ThreadPoolTaskExecutor.class)
2. @Configuration(proxyBeanMethods = false)
3. @EnableConfigurationProperties(TaskExecutionProperties.class)
4. public class TaskExecutionAutoConfiguration {
7.    /**
8.     * Bean name of the application {@link TaskExecutor}.
9.     */
10.    public static final String APPLICATION_TASK_EXECUTOR_BEAN_NAME = "applicationTaskExecutor";
13.    @Bean
14.    @ConditionalOnMissingBean
15.    public TaskExecutorBuilder taskExecutorBuilder(TaskExecutionProperties properties,
16.          ObjectProvider<TaskExecutorCustomizer> taskExecutorCustomizers,
17.          ObjectProvider<TaskDecorator> taskDecorator) {
18.       TaskExecutionProperties.Pool pool = properties.getPool();
19.       TaskExecutorBuilder builder = new TaskExecutorBuilder();
20.       builder = builder.queueCapacity(pool.getQueueCapacity());
21.       builder = builder.corePoolSize(pool.getCoreSize());
22.       builder = builder.maxPoolSize(pool.getMaxSize());
23.       builder = builder.allowCoreThreadTimeOut(pool.isAllowCoreThreadTimeout());
24.       builder = builder.keepAlive(pool.getKeepAlive());
25.       Shutdown shutdown = properties.getShutdown();
26.       builder = builder.awaitTermination(shutdown.isAwaitTermination());
27.       builder = builder.awaitTerminationPeriod(shutdown.getAwaitTerminationPeriod());
28.       builder = builder.threadNamePrefix(properties.getThreadNamePrefix());
29.       builder = builder.customizers(taskExecutorCustomizers.orderedStream()::iterator);
30.       builder = builder.taskDecorator(taskDecorator.getIfUnique());
31.       return builder;
32.    }
35.    @Lazy
36.    @Bean(name = { APPLICATION_TASK_EXECUTOR_BEAN_NAME,
37.          AsyncAnnotationBeanPostProcessor.DEFAULT_TASK_EXECUTOR_BEAN_NAME })
38.    @ConditionalOnMissingBean(Executor.class)
39.    public ThreadPoolTaskExecutor applicationTaskExecutor(TaskExecutorBuilder builder) {
40.       return builder.build();
41.    }
44. }

复制代码

代码分析：
  

• 这个设置类，欺压要求有ThreadPoolTaskExecutor才可以初始化
  
• TaskExecutionProperties：spring.task.execution开头的设置项，我们可以基于此举行定制化
  
• 通过建造者模式，使用TaskExecutorBuilder举行的ThreadPoolTaskExecutor封装构建。
  

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