针对 ThreadLocal 不可继承的特点，Java 还引入了 InheritableThreadLocal 类，顾名思义其是可以继承的
abstract.jpeg
基本实践
ThreadLocal 的不可继承性指的是子线程无法继承获取到父线程中的值。而 InheritableThreadLocal 则通过继承 ThreadLocal 类实现了可继承性。测试代码如下所示
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| @FixMethodOrder(MethodSorters.NAME_ASCENDING)
public class InheritableThreadLocalDemo {
@Test
public void test1() {
System.out.println("\n--------------------------- Test 1 ---------------------------");
ThreadLocal<String> userInfo = new ThreadLocal<>();
userInfo.set("Aaron");
new Thread( ()-> System.out.println("<子线程> userInfo: " + userInfo.get()) )
.start();
System.out.println("<父线程> userInfo: " + userInfo.get());
}
@Test
public void test2() {
System.out.println("\n--------------------------- Test 2 ---------------------------");
InheritableThreadLocal<String> userInfo = new InheritableThreadLocal<>();
userInfo.set("Aaron");
new Thread( ()-> System.out.println("<子线程> userInfo: " + userInfo.get()) )
.start();
System.out.println("<父线程> userInfo: " + userInfo.get());
}
}
|
测试结果如下，符合预期
figure 1.jpeg
基本原理
其基本原理也很简单，该类继承了 ThreadLocal 并重写了下述的三个方法。众所周知，ThreadLocal 是通过 Thread 类中 ThreadLocal.ThreadLocalMap 类型的 threadLocals 字段实现线程本地存储的。而 InheritableThreadLocal 则是使用 Thread 类中 ThreadLocal.ThreadLocalMap 类型的 inheritableThreadLocals 字段。其实现如下
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| 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);
}
}
|
而其继承性的实现也很简单，即在子线程创建阶段通过 init 方法将将父线程的 inheritableThreadLocals 浅拷贝到子线程中，Thread 实例创建过程中相关源码部分实现如下
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| public class Thread implements Runnable {
public Thread(Runnable target) {
init(null, target, "Thread-" + nextThreadNum(), 0);
}
private void init(ThreadGroup g, Runnable target, String name, long stackSize) {
init(g, target, name, stackSize, null, true);
}
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc, boolean inheritThreadLocals) {
...
Thread parent = currentThread();
...
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
...
}
}
|
前面提到子线程虽然继承了父线程的值，但其是通过浅拷贝的方式进行的。下面通过示例代码来验证下
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| @FixMethodOrder(MethodSorters.NAME_ASCENDING)
public class InheritableThreadLocalDemo {
@Test
public void test3() throws Exception {
System.out.println("\n--------------------------- Test 3 ---------------------------");
InheritableThreadLocal<String> userInfo = new InheritableThreadLocal<>();
userInfo.set("Aaron");
System.out.println("<父线程> userInfo #1: " + userInfo.get());
Thread thread = new Thread( ()-> {
System.out.println("<子线程> userInfo #2: " + userInfo.get());
userInfo.set("Bob");
System.out.println("<子线程> userInfo #3: " + userInfo.get());
} );
thread.start();
thread.join();
System.out.println("<父线程> userInfo #4: " + userInfo.get());
}
@Test
public void test4() throws Exception {
System.out.println("\n--------------------------- Test 4 ---------------------------");
InheritableThreadLocal<User> userInfo = new InheritableThreadLocal<>();
userInfo.set( new User("Aaron") );
System.out.println("<父线程> userInfo #1: " + userInfo.get());
Thread thread = new Thread( ()-> {
System.out.println("<子线程> userInfo #2: " + userInfo.get());
userInfo.get().setName("Bob");
System.out.println("<子线程> userInfo #3: " + userInfo.get());
} );
thread.start();
thread.join();
System.out.println("<父线程> userInfo #4: " + userInfo.get());
}
@AllArgsConstructor
@Data
private static class User {
private String name;
}
}
|
测试结果如下，符合预期
figure 2.jpeg
参考文献
- Java 并发编程之美 翟陆续、薛宾田著
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