一、java子線程中獲取父線程的threadLocal中的值
threadLocal可以做線程級的數據隔離，那如何在子線程中獲取父線程的值呢？ 可以使用InheritableThreadLocal
/**
* Java子線程獲取父線程的threadLocal中的值
*/
public class ThreadDemo1 {
public static final InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();
public static final ThreadLocal<String> threadLocal = new ThreadLocal<>();
public static void main(String[] args) throws Exception {
inheritableThreadLocal.set("inheritableThreadLocal hello");
threadLocal.set("threadLocal world");
new Thread(()->{
System.out.println(String.format("子線程可繼承值：%s", inheritableThreadLocal.get()));
System.out.println(String.format("子線程值：%s", threadLocal.get()));
new Thread(()->{
System.out.println(String.format("孫線程可繼承值：%s", inheritableThreadLocal.get()));
System.out.println(String.format("孫線程值：%s", threadLocal.get()));
}).start();
}).start();
Thread.sleep(3000);
System.out.println("--------------修改可繼承值threadLocal和threadLocal的值--------------------");
inheritableThreadLocal.set("inheritableThreadLocal hello2");
threadLocal.set("threadLocal world2");
new Thread(()->{
System.out.println(String.format("子線程可繼承值：%s", inheritableThreadLocal.get()));
System.out.println(String.format("子線程值：%s", threadLocal.get()));
new Thread(()->{
System.out.println(String.format("孫線程可繼承值：%s", inheritableThreadLocal.get()));
System.out.println(String.format("孫線程值：%s", threadLocal.get()));
}).start();
}).start();
}
}
輸出結果如下：
子線程可繼承值：inheritableThreadLocal hello 子線程值：null 孫線程可繼承值：inheritableThreadLocal hello 孫線程值：null --------------修改可繼承值threadLocal和threadLocal的值-------------------- 子線程可繼承值：inheritableThreadLocal hello2 子線程值：null 孫線程可繼承值：inheritableThreadLocal hello2 孫線程值：null
說明在子線程和孫線程中可以獲取到父線程的 inheritableThreadLocal 的值。修改inheritableThreadLocal 的值后，子線程和孫線程中同樣可以獲取到父線程的inheritableThreadLocal 的值。
但是threadLocal卻不行，獲取到的值為空。
2、查看創建Thread的源碼
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);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID();
}
//ThreadLocal.createInheritedMap
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
可以看到，創建線程時，調用了init方法，然后將父線程的inheritableThreadLocals賦值inheritableThreadLocals （ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;）