一、概述
为了方便且更加安全的进行多线程编程，jdk引入ThreadLocal和InheritableThreadLocal两个类，以供开发人员进行多线程之间的数据传递和数据共享。InheritableThreadLocal是ThreadLocal的子类，它可以实现子线程共享父线程的变量。
二、案例介绍
ThreadLocal：
private static ThreadLocal<String> testThreadLocal = new ThreadLocal<>();
/*
// 创建时可重写初始化方法
ThreadLocal<String> testThreadLocal = new ThreadLocal<String>(){
public Connection initialValue(){
return "zhangsan";
}
};
*/
public static void main(String[] args) {
// 设置线程变量
testThreadLocal.set("zhangsan");
// 获取线程变量
String userName = testThreadLocal.get();
System.out.println("userName: " + userName);
// 删除线程变量
testThreadLocal.remove();
userName = testThreadLocal.get();
System.out.println("userName: " + userName);
}
#结果输出
userName: zhangsan
userName: nullpublic static void main(String[] args) {
// 主线程
testThreadLocal.set("zhangsan");
System.out.println("userName0: " + testThreadLocal.get());
// 线程1
new Thread(() -> {
testThreadLocal.set("lisi");
System.out.println("userName1: " + testThreadLocal.get());
}).start();
// 线程2
new Thread(() -> {
testThreadLocal.set("wangwu");
System.out.println("userName2: " + testThreadLocal.get());
}).start();
}
#结果输出【线程之间变量相关隔离】
userName0: zhangsan
userName1: lisi
userName2: wangwu接下来看下set方法：
public void set(T value) {
// 获取当前线程(调用方线程：主线程、线程1......)
Thread t = Thread.currentThread();
// 当前线程作为key，获取对应的线程变量ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null) {
// 设置线程变量：key为当前定义的ThreadLocal实例的this引用，值为我们传入的数据
map.set(this, value);
} else {
// 第一次设置线程变量，则会创建ThreadLocalMap
createMap(t, value);
}
}再看下get方法：
public T get() {
// 获取当前线程(调用方线程：主线程、线程1......)
Thread t = Thread.currentThread();
// 当前线程作为key，获取对应的线程变量ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null) {
// 值最终是存在Entry对象的value属性
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
T result = (T)e.value;
return result;
}
}
// ThreadLocalMap为空，则初始化操作
return setInitialValue();
}
private T setInitialValue() {
// 创建ThreadLocal时可重写初始化方法
T value = initialValue();
// 获取当前线程(调用方线程：主线程、线程1......)
Thread t = Thread.currentThread();
// 当前线程作为key，获取对应的线程变量ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null) {
// 设置线程变量：key为当前定义的ThreadLocal实例的this引用，值为初始化方法返回的数据
map.set(this, value);
} else {
// 第一次设置线程变量，则会创建ThreadLocalMap
createMap(t, value);
}
return value;
}InheritableThreadLocal：
private static InheritableThreadLocal<String> testInheritableThreadLocal = new InheritableThreadLocal<>();
public static void main(String[] args) {
// 主线程
testInheritableThreadLocal.set("zhangsan");
System.out.println("userName0: " + testInheritableThreadLocal.get());
// 线程1
new Thread(() -> System.out.println("userName1: " + testInheritableThreadLocal.get())).start();
// 线程2
new Thread(() -> System.out.println("userName2: " + testInheritableThreadLocal.get())).start();
}
#结果输出
userName0: zhangsan
userName1: zhangsan
userName2: zhangsan查看InheritableThreadLocal的源码：
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继承了ThreadLocal类型，并且重写了getMap和createMap方法，唯一的区别是：threadLocals(ThreadLocalMap类型)变成了inheritableThreadLocals(ThreadLocalMap类型)。
查看get方法：
public T get() {
Thread t = Thread.currentThread();
// 注意：InheritableThreadLocal重写了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设置的地方，最终定位到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)
// 将父线程inheritableThreadLocals复制给子线程inheritableThreadLocals
// 此处可联想到：如果使用了线程池，而线程池中的线程是复用的，不会再次调用初始化方法
// 所以无法将父线程inheritableThreadLocals复制给子线程inheritableThreadLocals
this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
this.stackSize = stackSize;
tid = nextThreadID();
}三、注意事项
- 使用ThreadLocal、或者InheritableThreadLocal方法时，注意及时调用remove方法进行清理。
- 注意线程池结合InheritableThreadLocal的使用，线程池中的线程是复用的，不会再次调用初始化方法，所以无法将父线程inheritableThreadLocals复制给子线程inheritableThreadLocals。
