ThreadLocal和InheritableThreadLocal詳解
作者：言沫東
為了方便且更加安全的進行多線程編程，jdk引入ThreadLocal和InheritableThreadLocal兩個類，以供開發人員進行多線程之間的數據傳遞和數據共享。InheritableThreadLocal是ThreadLocal的子類，它可以實現子線程共享父線程的變量。
一、概述
為了方便且更加安全的進行多線程編程，jdk引入ThreadLocal和InheritableThreadLocal兩個類，以供開發人員進行多線程之間的數據傳遞和數據共享。InheritableThreadLocal是ThreadLocal的子類，它可以實現子線程共享父線程的變量。
二、案例介紹
ThreadLocal：
private static ThreadLocal<String> testThreadLocal = new ThreadLocal<>();
/*
// 創建時可重寫初始化方法
ThreadLocal<String> testThreadLocal = new ThreadLocal<String>(){
public Connection initialValue(){
return "zhangsan";
}
};
*/
public static void main(String[] args) {
// 設置線程變量
testThreadLocal.set("zhangsan");
// 獲取線程變量
String userName = testThreadLocal.get();
System.out.println("userName: " + userName);
// 刪除線程變量
testThreadLocal.remove();
userName = testThreadLocal.get();
System.out.println("userName: " + userName);
}
#結果輸出
userName: zhangsan
userName: nullpublic static void main(String[] args) {
// 主線程
testThreadLocal.set("zhangsan");
System.out.println("userName0: " + testThreadLocal.get());
// 線程1
new Thread(() -> {
testThreadLocal.set("lisi");
System.out.println("userName1: " + testThreadLocal.get());
}).start();
// 線程2
new Thread(() -> {
testThreadLocal.set("wangwu");
System.out.println("userName2: " + testThreadLocal.get());
}).start();
}
#結果輸出【線程之間變量相關隔離】
userName0: zhangsan
userName1: lisi
userName2: wangwu接下來看下set方法：
public void set(T value) {
// 獲取當前線程(調用方線程：主線程、線程1......)
Thread t = Thread.currentThread();
// 當前線程作為key，獲取對應的線程變量ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null) {
// 設置線程變量：key為當前定義的ThreadLocal實例的this引用，值為我們傳入的數據
map.set(this, value);
} else {
// 第一次設置線程變量，則會創建ThreadLocalMap
createMap(t, value);
}
}再看下get方法：
public T get() {
// 獲取當前線程(調用方線程：主線程、線程1......)
Thread t = Thread.currentThread();
// 當前線程作為key，獲取對應的線程變量ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null) {
// 值最終是存在Entry對象的value屬性
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
T result = (T)e.value;
return result;
}
}
// ThreadLocalMap為空，則初始化操作
return setInitialValue();
}
private T setInitialValue() {
// 創建ThreadLocal時可重寫初始化方法
T value = initialValue();
// 獲取當前線程(調用方線程：主線程、線程1......)
Thread t = Thread.currentThread();
// 當前線程作為key，獲取對應的線程變量ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null) {
// 設置線程變量：key為當前定義的ThreadLocal實例的this引用，值為初始化方法返回的數據
map.set(this, value);
} else {
// 第一次設置線程變量，則會創建ThreadLocalMap
createMap(t, value);
}
return value;
}InheritableThreadLocal：
private static InheritableThreadLocal<String> testInheritableThreadLocal = new InheritableThreadLocal<>();
public static void main(String[] args) {
// 主線程
testInheritableThreadLocal.set("zhangsan");
System.out.println("userName0: " + testInheritableThreadLocal.get());
// 線程1
new Thread(() -> System.out.println("userName1: " + testInheritableThreadLocal.get())).start();
// 線程2
new Thread(() -> System.out.println("userName2: " + testInheritableThreadLocal.get())).start();
}
#結果輸出
userName0: zhangsan
userName1: zhangsan
userName2: zhangsan查看InheritableThreadLocal的源碼：
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);
}
}InheritableThreadLocal繼承了ThreadLocal類型，并且重寫了getMap和createMap方法，唯一的區別是：threadLocals(ThreadLocalMap類型)變成了inheritableThreadLocals(ThreadLocalMap類型)。
查看get方法：
public T get() {
Thread t = Thread.currentThread();
// 注意：InheritableThreadLocal重寫了getMap方法，返回inheritableThreadLocals
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}查看inheritableThreadLocals設置的地方，最終定位到java.lang.Thread#init方法：
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;
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null) {
if (security != null) {
g = security.getThreadGroup();
}
if (g == null) {
g = parent.getThreadGroup();
}
}
g.checkAccess();
if (security != null) {
if (isCCLOverridden(getClass())) {
security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
if (security == null || isCCLOverridden(parent.getClass()))
this.contextClassLoader = parent.getContextClassLoader();
else
this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext =
acc != null ? acc : AccessController.getContext();
this.target = target;
setPriority(priority);
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
// 將父線程inheritableThreadLocals復制給子線程inheritableThreadLocals
// 此處可聯想到：如果使用了線程池，而線程池中的線程是復用的，不會再次調用初始化方法
// 所以無法將父線程inheritableThreadLocals復制給子線程inheritableThreadLocals
this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
this.stackSize = stackSize;
tid = nextThreadID();
}三、注意事項
- 使用ThreadLocal、或者InheritableThreadLocal方法時，注意及時調用remove方法進行清理。
- 注意線程池結合InheritableThreadLocal的使用，線程池中的線程是復用的，不會再次調用初始化方法，所以無法將父線程inheritableThreadLocals復制給子線程inheritableThreadLocals。
責任編輯：姜華
來源：
今日頭條
