通过ThreadLocal和InheritableThreadLocal,我们能够很方便的设计出线程安全的类。JDK底层是如何做到的呢?ThreadLocal和InheritableThreadLocal有什么区别呢与联系呢?为什么有了ThreadLocal类还需要InheritableThreadLocal类,他们与Thread类是什么关系?带着这些问题我们来分析他们的源码。
ThreadLocal
ThreadLocal是为每一个线程创建一个单独的变量副本,每个线程都可以独立地改变自己所拥有的变量副本,而不会影响其他线程所对应的副本。ThreadLocal为多线程环境下变量问题提供了另外一种解决思路。下面来看看ThreadLocal内部是如何做到的。下面分别分析ThreadLocal提供的方法。
- set方法源码解析
public class ThreadLocal<T> {
/**
**传入一个value参数
**/
public void set(T value) {
//首先得到当前线程对象
Thread t = Thread.currentThread();
//根据当前线程,得到ThreadLocalMap 引用。
ThreadLocalMap map = getMap(t);
//如果map不为空,则把值set到map里
if (map != null)
map.set(this, value);
else
//map为空,则创建一个map对象,并设置值
createMap(t, value);
}
//根据Thread得到 ThreadLocalMap 对象
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
//map为空的话,会新建一个ThreadLocalMap,并将引用交给当前Thread的threadLocals 变量
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
}
从上面的源码上我们可以很容易的看出来,** ThreadLocal只是提供方法方便我们进行编码,而真正存变量的地方是在ThreadLocalMap 这个对象上的**。Thread对象持有ThreadLocalMap 的引用,ThreadLocalMap 以ThreadLocal为key。他们之间的关系如下图所示:- get方法源码解析
public class ThreadLocal<T> {
public T get() {
//得到当前线程
Thread t = Thread.currentThread();
//这里取的是Thread里的ThreadLocalMap引用
ThreadLocalMap map = getMap(t);
if (map != null) {
//从map里取值,key为ThreadLocal对象
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
//map为空的话,会调用setInitialValue方法
return setInitialValue();
}
//
private T setInitialValue() {
//得到一个默认的值,initialValue为protected,留给子类实现,
T value = initialValue();
//得到当前线程
Thread t = Thread.currentThread();
//再次尝试得到map对象
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
//调用createMap方法
createMap(t, value);
return value;
}
protected T initialValue() {
return null;
}
}
get方法无非就是得到当前线程的ThreadLocalMap,如果不为空,则根据ThreadLocal为key进行取值,如果为空,则会调用createMap为Thread线程构造一个ThreadLocalMap对象,并返回initialValue方法的值。
InheritableThreadLocal
InheritableThreadLocal用于子线程能够拿到父线程往ThreadLocal里设置的值。使用代码如下:
public class Test {
public static ThreadLocal<Integer> threadLocal = new InheritableThreadLocal<Integer>();
public static void main(String args[]) {
threadLocal.set(new Integer(456));
Thread thread = new MyThread();
thread.start();
System.out.println("main = " + threadLocal.get());
}
static class MyThread extends Thread {
@Override
public void run() {
System.out.println("MyThread = " + threadLocal.get());
}
}
}
输出结果如下图:如果把上面的InheritableThreadLocal换成ThreadLocal的话,在子线程里的输出将为是空。从上面的代码里也可以看出InheritableThreadLocal是ThreadLocal的子类。下面是InheritableThreadLocal的源码:
package java.lang;
import java.lang.ref.*;
public class InheritableThreadLocal<T> extends ThreadLocal<T> {
//这个方法留给子类实现
protected T childValue(T parentValue) {
return parentValue;
}
//重写getMap方法,返回的是Thread的inheritableThreadLocals引用
ThreadLocalMap getMap(Thread t) {
return t.inheritableThreadLocals;
}
//重写createMap方法,构造的ThreadLocalMap会传给Thread的inheritableThreadLocals 变量
void createMap(Thread t, T firstValue) {
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}
}
从源码上看,跟ThreadLocal不一样的无非是ThreadLocalMap的引用不一样了,从逻辑上来讲,这并不能做到子线程得到父线程里的值。那秘密在那里呢?通过跟踪Thread的构造方法,你能够发现是在构造Thread对象的时候对父线程的InheritableThreadLocal进行了复制。下面是Thread的部分源码:
public class Thread implements Runnable {
//默认人构造方法,会调用init方法进行初使化
public Thread() {
init(null, null, "Thread-" + nextThreadNum(), 0);
}
private void init(ThreadGroup g, Runnable target, String name,
long stackSize) {
init(g, target, name, stackSize, null);
}
//最终会调用到当前这个方法
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc) {
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name.toCharArray();
// parent为当前线程,也就是调用了new Thread();方法的线程
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;
//在这里会继承父线程是否为后台线程的属性还有父线程的优先级
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);
//这里是重点,当父线程的inheritableThreadLocals 不为空的时候,会调用 ThreadLocal.createInheritedMap方法,传入的是父线程的inheritableThreadLocals。原来复制变量的秘密在这里
if (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();
}
}
通过跟踪Thread的构造方法,我们发现只要父线程在构造子线程(调用new Thread())的时候inheritableThreadLocals变量不为空。新生成的子线程会通过ThreadLocal.createInheritedMap方法将父线程inheritableThreadLocals变量有的对象复制到子线程的inheritableThreadLocals变量上。这样就完成了线程间变量的继承与传递。
ThreadLocal.createInheritedMap方法的源码如下:
public class ThreadLocal<T> {
//根据传入的map,构造一个新的ThreadLocalMap
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
static class ThreadLocalMap {
//这个private的构造方法就是专门给ThreadLocal使用的
private ThreadLocalMap(ThreadLocalMap parentMap) {
//ThreadLocalMap还是用Entry数组来存储对象的
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new Entry[len];
//这里是复制parentMap数据的逻辑
for (int j = 0; j < len; j++) {
Entry e = parentTable[j];
if (e != null) {
ThreadLocal key = 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++;
}
}
}
}
}
}
总结:
- ThreadLocal和InheritableThreadLocal本质上只是为了方便编码给的工具类,具体存数据是ThreadLocalMap 对象。
- ThreadLocalMap 存的key对象是ThreadLocal,value就是真正需要存的业务对象。
- Thread里通过两个变量持用ThreadLocalMap 对象,分别为:threadLocals和inheritableThreadLocals。
- InheritableThreadLocal之所以能够完成线程间变量的传递,是在new Thread()的时候对inheritableThreadLocals对像里的值进行了复制。
- 子线程通过继承得到的InheritableThreadLocal里的值与父线程里的InheritableThreadLocal的值具有相同的引用,如果父子线程想实现不影响各自的对象,可以重写InheritableThreadLocal的childValue方法。