在系统编码中，有时候我们需要在不同的情况下，从当前线程获取某些值，这就需要在特定时候为线程设置值。比如说：
- 在当前线程设置值
- 在当前线程新创建的线程设置值
- 在当前线程所复用线程池中的某个线程设置值
线程在java中就是用Thread类来表示的。其中有俩个属性threadLocals、inheritableThreadLocals是用来存储该线程从其他地方获取，分别是由ThreadLocal、InheritableThreadLocal类来维护。这个俩个参数分别表示当前线程为自己设置的值、
从父线程中设置的值。以下是一个Thread类对俩个属性的表示图：
然后再看一看，一个Thread创建过程中跟上下文参数相关的事：
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| private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
}
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ThreadLocal
ThreadLocal是表示当前线程本地的变量，通常用于在某个线程的业务生命周期中设置数据、获取数据，比如说：后端系统常用的UserContext。
原理
ThreadLocal实现的原理就是通过获取当前线程，然后获取该线程本身的threadLocals变量，后面的操作都是针对这个变量来操作的。
1、get源码
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| public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
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2、set源码
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| public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
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ThreadLocal的缺点
只能在当前线程生效，如果你在当前线程新创建的一个线程，新的线程就无法获取父线程设置的值。比如：
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| public class ThreadLocalDefectDemo {
private static ThreadLocal<String> threadLocal = new ThreadLocal<>();
public static void main(String[] args) {
threadLocal.set("threadLocal做不到的事");
new Thread(() -> {
System.out.println(threadLocal.get());
}).start();
}
}
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拿不到值的原因就是，ThreadLocal是与Thread相绑定的，新创建的线程所绑定的ThreadLocal是获取不到父线程绑定的ThreadLocal，而且在创建新线程时，也不会去复制父线程的threadLocals。
InheritableThreadLocal
InheritableThreadLocal是ThreadLocal的子类，它表示的是会传递给子线程的数据。
原理
InheritableThreadLocal的原理就是让新创建的线程复制父线程的inheritableThreadLocals到新线程的inheritableThreadLocals中，然后覆盖ThreadLocal三个方法，让线程在获取值时，是从当前线程的inheritableThreadLocals获取数据。
InheritableThreadLocal的缺点
InheritableThreadLocal虽然可以可以解决父子线程获取值的问题。但它无法在线程池复用中，获取父线程的数据。原因就是inheritableThreadLocals是仅在创建线程中才去复制数据，而线程池中的线程是已经创建好的，也就没有从父线程复制数据的过程了。
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| public class InheritableThreadLocalDefectDemo {
private static InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();
public static void main(String[] args) throws ExecutionException, InterruptedException {
inheritableThreadLocal.set("threadLocal做不到的事");
new Thread(() -> {
System.out.println(inheritableThreadLocal.get());
}).start();
ThreadFactory namedThreadFactory = new ThreadFactoryBuilder().setNameFormat("ProcessMessage-pool-%d").build();
ExecutorService executorService = new ThreadPoolExecutor(1, 1, 2000,
TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<>(200),
namedThreadFactory,
new ThreadPoolExecutor.AbortPolicy());
Future<?> submit = executorService.submit(() -> {
System.out.println(inheritableThreadLocal.get());
inheritableThreadLocal.set(null);
});
Object o = submit.get();
executorService.submit(() -> {
System.out.println(inheritableThreadLocal.get());
});
Object o1 = submit.get();
executorService.shutdown();
}
}
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TransmittableThreadLocal
TransmittableThreadLocal是阿里巴巴开源出来的第三方库，专门用来解决线程复用的问题。
原理
TransmittableThreadLocal是在线程池提交任务之前将父线程的数据拷贝一份，然后在线程执行前，将这份拷贝数据复制到该线程的inheritableThreadLocals中。
具体流程图如下：
具体代码如下：
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| public class TransmittableThreadLocalDemo {
private static TransmittableThreadLocal<String> transmittableThreadLocal = new TransmittableThreadLocal<>();
public static void main(String[] args) throws ExecutionException, InterruptedException {
transmittableThreadLocal.set("全都能看见");
ThreadFactory namedThreadFactory = new ThreadFactoryBuilder().setNameFormat("ProcessMessage-pool-%d").build();
ExecutorService executorService = new ThreadPoolExecutor(1, 1, 2000,
TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<>(200),
namedThreadFactory,
new ThreadPoolExecutor.AbortPolicy());
ExecutorService ttlExecutorService = TtlExecutors.getTtlExecutorService(executorService);
Future<?> submit = ttlExecutorService.submit(() -> {
System.out.println(transmittableThreadLocal.get());
transmittableThreadLocal.set(null);
});
Object o = submit.get();
transmittableThreadLocal.set("变一下吧");
ttlExecutorService.submit(() -> {
System.out.println(transmittableThreadLocal.get());
});
Object o1 = submit.get();
ttlExecutorService.shutdown();
}
}
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## 对Spring的ThreadPoolTaskExecutor改造
其他问题
数据拷贝问题
InheritableThreadLocal、TransmittableThreadLocal在拷贝父线程数据时，默认都是采用浅拷贝的方式。比如说：
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| public class InheritableThreadLocalDefectDataDemo {
private static InheritableThreadLocal<UserInfo> infoInheritableDataThreadLocal = new InheritableThreadLocal<>();
public static void main(String[] args) throws ExecutionException, InterruptedException {
UserInfo userInfo = UserInfo.builder()
.normalMessage("normalMessage")
.userDetailInfo(UserDetailInfo.builder().innerMessage("innerMessage").build())
.build();
infoInheritableDataThreadLocal.set(userInfo);
ThreadFactory namedThreadFactory = new ThreadFactoryBuilder().setNameFormat("ProcessMessage-pool-%d").build();
ExecutorService executorService = new ThreadPoolExecutor(1, 1, 2000,
TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<>(200),
namedThreadFactory,
new ThreadPoolExecutor.AbortPolicy());
Future<?> submit = executorService.submit(() -> {
System.out.println(infoInheritableDataThreadLocal.get().getNormalMessage());
System.out.println(infoInheritableDataThreadLocal.get().getUserDetailInfo().getInnerMessage());
infoInheritableDataThreadLocal.get().setUserDetailInfo(null);
});
Object o = submit.get();
System.out.println(infoInheritableDataThreadLocal.get().getUserDetailInfo());
executorService.shutdown();
}
}
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如果你希望采用深拷贝来拷贝数据，就得自己写一个类继承InheritableThreadLocal并重写childValue方法。例如：
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| public class InheritableThreadLocalDefectDataDemo {
private static InheritableThreadLocal<UserInfo> infoInheritableDataThreadLocal = new InheritableThreadLocal<>();
private static MyInheritableThreadLocal<UserInfo> myInheritableThreadLocal = new MyInheritableThreadLocal<>();
public static void main(String[] args) throws ExecutionException, InterruptedException {
UserInfo userInfo = UserInfo.builder()
.normalMessage("normalMessage")
.userDetailInfo(UserDetailInfo.builder().innerMessage("innerMessage").build())
.build();
UserInfo userInfo2 = UserInfo.builder()
.normalMessage("normalMessage")
.userDetailInfo(UserDetailInfo.builder().innerMessage("innerMessage").build())
.build();
infoInheritableDataThreadLocal.set(userInfo2);
myInheritableThreadLocal.set(userInfo);
ThreadFactory namedThreadFactory = new ThreadFactoryBuilder().setNameFormat("ProcessMessage-pool-%d").build();
ExecutorService executorService = new ThreadPoolExecutor(1, 1, 2000,
TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<>(200),
namedThreadFactory,
new ThreadPoolExecutor.AbortPolicy());
Future<?> submit = executorService.submit(() -> {
System.out.println(infoInheritableDataThreadLocal.get().getNormalMessage());
System.out.println(infoInheritableDataThreadLocal.get().getUserDetailInfo().getInnerMessage());
System.out.println(myInheritableThreadLocal.get().getUserDetailInfo().getInnerMessage());
infoInheritableDataThreadLocal.get().setUserDetailInfo(null);
myInheritableThreadLocal.get().setUserDetailInfo(null);
});
Object o = submit.get();
System.out.println(infoInheritableDataThreadLocal.get().getUserDetailInfo());
System.out.println(myInheritableThreadLocal.get().getUserDetailInfo());
executorService.shutdown();
}
}
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https://github.com/alibaba/transmittable-thread-local
https://mp.weixin.qq.com/s/a6IGrOtn1mi0r05355L5Ng
https://blog.csdn.net/prestigeding/article/details/54945658
文中代码：https://github.com/wangjie-fourth/CodeDemo/tree/main/ttl