首先我们看一个例子
public class TestThreadLocal {
public static ThreadLocal<String> threadLocal = new ThreadLocal<>();
public static void main( String[] args ) {
// 色后置线程变量
threadLocal.set("hello world");
Thread thread = new Thread(new Runnable() {
@Override
public void run( ) {
//子线程输出线程变量的值
System.out.println("thread:"+threadLocal.get());
}
});
thread.start();
// 主线程输出线程变量的值
System.out.println("main:"+threadLocal.get());
}
}
输出结果如下:
main:hello world
thread:null
也就是说,同一个ThreadLocal变量在父线程中被设置后,在子线程中是获取不到的;
原因: 因为在子线程thread里面调用get方法时当前线程为thread线程,而这里调用set方法设置线程变量的是main线程,两者是不同的线程,自然子线程访问时返回null,那么有没有办法让子线程能访问到父线程中的值?答案是有!
为了解决上面的问题,InheritableThreadLocal应运而生,InheritableThreadLocal继承ThreadLocal,其提供一个特性,就是让子线程可以访问在父线程中设置的本地变量,我们上面代码修改为InheritableThreadLocal
;
public class TestThreadLocal {
public static InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
public static void main( String[] args ) {
// 色后置线程变量
threadLocal.set("hello world");
Thread thread = new Thread(new Runnable() {
@Override
public void run( ) {
//子线程输出线程变量的值
System.out.println("thread:"+threadLocal.get());
}
});
thread.start();
// 主线程输出线程变量的值
System.out.println("main:"+threadLocal.get());
}
}
输出结果
main:hello world
thread:hello world
下面我们看一下InheritableThreadLocal的原理:
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);
}
}
在InheritableThreadLocal的世界里,变量inheritableThreadLocals 替代了threadLocals;
那么如何让子线程可以访问父线程的本地变量。这要从创建Thread的代码说起,打开Thread类的默认构造方法,代码如下:
public Thread(Runnable target) {
init(null, target, "Thread-" + nextThreadNum(), 0);
}
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name;
//获取当前线程
Thread parent = currentThread();
//如果父线程的 inheritableThreadLocals变量不为null
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
//设置子线程inheritThreadLocals变量
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID();
}
我们看下createInheritedMap代码:
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
在createInheritedMap内部使用父线程的inheritableThreadLocals变量作为构造方法创建了一个新的ThreadLocalMap变量,然后赋值给子线程的inheritableThreadLocals变量。下面看看在ThreadLocalMap的构造函数内部做了什么事情;
private ThreadLocalMap(ThreadLocalMap parentMap) {
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new Entry[len];
for (int j = 0; j < len; j++) {
Entry e = parentTable[j];
if (e != null) {
@SuppressWarnings("unchecked")
ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
if (key != null) {
Object value = key.childValue(e.value);
Entry c = new Entry(key, value);
int h = key.threadLocalHashCode & (len - 1);
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
}
在该构造函数内部把父线程的inheritableThreadLocals成员变量的值复制到新的ThreadLocalMap对象中。
总结
InheritableThreadLocal 类通过重写下面代码
ThreadLocalMap getMap(Thread t) {
return t.inheritableThreadLocals;
}
/**
* Create the map associated with a ThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the table.
*/
void createMap(Thread t, T firstValue) {
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}
让本地变量保存到了具体的线程的inheritableThreadLocals变量里面,那么线程在通过InheritableThreadLocal类实例的set或者get方法设置变量时,就会创建当前线程的inheritableThreadLocals变量。
当父线程创建子线程时,构造方法会把父线程中的inheritableThreadLocals变量里面的本地变量赋值一份保存到子线程的inheritableThreadLocals变量里面。
那么什么情况下需要子线程可以获取父线程的threadlocal变量呢?
比如子线程需要使用存放在threadlocal变量中的用户登录信息,再比如一些中间件需要把统一的id追踪的 整个调用链记录下来。
其实子线程使用父线程中的threadlocal方法有多种方式,比如创建线程时传入父线程中的变量,并将其复制到子线程中,或者在父线程中构造一个map作为参数传递给子线程,但是这些都改变了我们的使用习惯,所以在这些情况下InheritableThreadLocal就显得比较有用。
史上最全的并发编程脑图:https://www.processon.com/view/5d43e6cee4b0e47199351b7f