一、ThreadLocal 在父子線程傳遞的問題

public class InheritableThreadLocalDemo {// 全局變量 // static ThreadLocal<String> threadLocal = new ThreadLocal<>();static ThreadLocal<String> threadLocal = new InheritableThreadLocal<>();public static void main(String[] args) {new Thread(new Runnable() {@Overridepublic void run() {InheritableThreadLocalDemo.threadLocal.set("superWorld");new Service().call();}}).start();} }class Service {public void call() {System.out.println("Service:" + Thread.currentThread().getName() + " : " + InheritableThreadLocalDemo.threadLocal.get());//new Dao().call();new Thread(new Runnable() {@Overridepublic void run() {new Dao().call();}}).start();} }class Dao {public void call() {System.out.println("Dao:" + Thread.currentThread().getName() + " : " + InheritableThreadLocalDemo.threadLocal.get());}}

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二、InheritableThreadLocal 和?ThreadLocal 的區別

1. InheritableThreadLocal 實現

public class InheritableThreadLocal<T> extends ThreadLocal<T> {/*** Computes the child's initial value for this inheritable thread-local* variable as a function of the parent's value at the time the child* thread is created. This method is called from within the parent* thread before the child is started.* <p>* This method merely returns its input argument, and should be overridden* if a different behavior is desired.** @param parentValue the parent thread's value* @return the child thread's initial value*/protected T childValue(T parentValue) {return parentValue;}/*** Get the map associated with a ThreadLocal.** @param t the current thread*/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);} }

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2. ThreadLocal 的getMap、createMap實現

/*** Get the map associated with a ThreadLocal. Overridden in* InheritableThreadLocal.** @param t the current thread* @return the map*/ThreadLocalMap getMap(Thread t) {return t.threadLocals;}/*** Create the map associated with a ThreadLocal. Overridden in* InheritableThreadLocal.** @param t the current thread* @param firstValue value for the initial entry of the map*/void createMap(Thread t, T firstValue) {t.threadLocals = new ThreadLocalMap(this, firstValue);}

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通過上面的代碼我們可以看到InheritableThreadLocal 重寫了childValue, getMap,createMap三個方法，

當我們往里面set值的時候，值保存到了inheritableThreadLocals里面，而不是之前的threadLocals。

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三、問題：為什么當創建子線程時，可以獲取到上個線程里的threadLocal中的值呢？

原因就是在新創建線程的時候，會把之前線程的inheritableThreadLocals賦值給新線程的inheritableThreadLocals，通過這種方式實現了數據的傳遞。

代碼關鍵點：

1. Thread#init

if (parent.inheritableThreadLocals != null)this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

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2.?ThreadLocal.createInheritedMap

　　static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {return new ThreadLocalMap(parentMap);}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++;}}}}

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