? ? ? ? 说到可继承的线程变量，大家可能会想到jdk里的实现java.lang.InheritableThreadLocal。它拥有和线程变量ThreadLocal一样的功能，并且，在当前线程上创建一个新的线程实例Thread时，会把这些线程变量从当前线程传递给新的线程实例。（此时线程变量不再线程安全，需要考虑线程安全问题）
InheritableThreadLocal:
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class="java" name="code">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.
* @param map the map to store.
*/
void createMap(Thread t, T firstValue) {
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}
}
?该类继承了ThreadLocal并重写了和ThreadLocalMap相关的方法。这个ThreadLocalMap其实是java线程对象Thread类的两个属性
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class Thread implements Runnable { ....... /* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ ThreadLocal.ThreadLocalMap threadLocals = null; /* * InheritableThreadLocal values pertaining to this thread. This map is * maintained by the InheritableThreadLocal class. */ ThreadLocal.ThreadLocalMap inheritableThreadLocals = null; ........ }
?这两个ThreadLocalMap其实就是线程变量实际存放的地方，我们用了线程变量set内容，其实就是往ThreadLocalMap里put内容，key是你定义的ThreadLocal本身，value是你往线程变量set的内容。因为内容是存在线程本身上，所以，同一个线程跨了多少个方法都可以访问到，不同线程就访问不到或访问到不同的对象，实现了线程安全。
? ?ThreadLocal的set方法：
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
?其中
ThreadLocalMap map = getMap(t);
?这行代码，ThreadLocal的getMap：
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
?而InheritableThreadLocal的getMap：
/**
* Get the map associated with a ThreadLocal.
*
* @param t the current thread
*/
ThreadLocalMap getMap(Thread t) {
return t.inheritableThreadLocals;
}
? ? 即InheritableThreadLocal重写了和ThreadLocalMap相关的方法，其实就是把set的内容放在线程对象的inheritableThreadLocals属性上， 而普通的ThreadLocal则是把set的内容放在线程对象的threadLocals属性上。我们平时新建一个线程，new Thread()，在Thread类实例化的时候调用init()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");
}
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null) {
/* Determine if it's an applet or not */
/* If there is a security manager, ask the security manager
what to do. */
if (security != null) {
g = security.getThreadGroup();
}
/* If the security doesn't have a strong opinion of the matter
use the parent thread group. */
if (g == null) {
g = parent.getThreadGroup();
}
}
/* checkAccess regardless of whether or not threadgroup is
explicitly passed in. */
g.checkAccess();
/*
* Do we have the required permissions?
*/
if (security != null) {
if (isCCLOverridden(getClass())) {
security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
this.name = name.toCharArray();
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 (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();
}
?其中
if (parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
?这行代码把当前线程对象的inheritableThreadLocals属性传递给新建的线程对象inheritableThreadLocals属性，即实现了线程变量的传递。
? ? 上面是可继承的线程变量inheritableThreadLocals的实现原理。但是在实际的应用场景里，绝大多数都是使用线程池来进行多线程编程，所以jdk提供的inheritableThreadLocals类实用性不高。在线程池(ThreadPoolExecutor)中运行一个Runable实例并不会去新建一个线程，而是把Runable实例添加到队列中(在核心线程数已实例化满的时候),让ThreadPoolExecutor的workers去从队列里拿出Runable实例（这是一个典型的生产者消费者模式），然后运行Runable实例.run()方法。故jdk的inheritableThreadLocals这种实现方式没法适用。
? ? ?所以我就想着写一个能在executor上传递的可继承线程变量。而要实现这个功能，单单线程变量本身是不够的，还需要线程池的配合。通过我以前写的博客 http://xiangshouxiyang.iteye.com/blog/2354074 《线程池增强实现》给的思路，设计了自定义的InheritableThreadLocal：
package com.hcd;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.CopyOnWriteArrayList;
/**
* 可在特定线程池中继承的线程变量（配合InheritableThreadLocalExecutor使用）
* Created by cd_huang on 2017/8/28.
*/
public class InheritableThreadLocal<T> extends ThreadLocal<T>{
private static List<InheritableThreadLocal> inheritableExecutorThreadLocalList =new CopyOnWriteArrayList<>();
public InheritableThreadLocal(){
this(true);
}
public InheritableThreadLocal(boolean isAdd){
/**
* 一般线程变量本身也不需要被垃圾回收
*/
if(isAdd){
inheritableExecutorThreadLocalList.add(this);
}
}
/**
* 从map里取出内容set线程变量（protected方法，可重写，但不提倡直接调用）
* @param map
*/
protected void setThreadLocalFromMap(Map map){
T obj = (T)map.get(this);
this.set(obj);
}
/**
* get线程变量装到map里（protected方法，可重写，但不提倡直接调用）
* @param map
*/
protected void getThreadLocalputMap(Map map){
T obj = this.get();
map.put(this,obj);
}
/**
* 移除掉线程变量（protected方法，可重写，但不提倡直接调用）
*/
protected void removeThreadLocal(){
this.remove();
}
/**
* 把当前线程可传递的线程变量内容放在map里，在task放进线程池队列前调用
* @return
*/
public static Map<Object,Object> getThreadLocalsMap(){
Map<Object,Object> threadLocalMap =new HashMap<>();
List<InheritableThreadLocal> list =inheritableExecutorThreadLocalList;
for(InheritableThreadLocal threadLocal:list){
threadLocal.getThreadLocalputMap(threadLocalMap);
}
return threadLocalMap;
}
/**
* 把map里的内容重新set线程变量内容，在task真正运行run方法前调用
* @param threadLocalMap
*/
public static void setThreadLocalsFromMap(Map<Object,Object> threadLocalMap){
List<InheritableThreadLocal> list =inheritableExecutorThreadLocalList;
for(InheritableThreadLocal threadLocal:list){
threadLocal.setThreadLocalFromMap(threadLocalMap);
}
}
/**
* 把setThreadLocalsFromMap方法set的线程变量内容清空，在task真正运行run方法后调用
*/
public static void removeThreadLocals(){
List<InheritableThreadLocal> list =inheritableExecutorThreadLocalList;
for(InheritableThreadLocal threadLocal:list){
threadLocal.removeThreadLocal();
}
}
}
?与之配合的InheritableThreadLocalExecutor：
package com.hcd;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.Map;
import java.util.concurrent.*;
/**
* 支持可继承的线程变量的线程池（配合InheritableThreadLocal使用）
* Created by cd_huang on 2017/8/29.
*/
public class InheritableThreadLocalExecutor extends ThreadPoolExecutor {
private static Logger logger = LoggerFactory.getLogger(InheritableThreadLocalExecutor.class);
public InheritableThreadLocalExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
}
public InheritableThreadLocalExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory);
}
public InheritableThreadLocalExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, handler);
}
public InheritableThreadLocalExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, handler);
}
/**
* 重写执行线程实例的方法
* @param command
*/
@Override
public void execute(Runnable command) {
if (command == null){
throw new NullPointerException();
}
TaskWithThreadLocal task =new TaskWithThreadLocal(command,InheritableThreadLocal.getThreadLocalsMap());
super.execute(task);
}
/**
* 新线程执行时带上指定的线程信息
* @param
*/
private static class TaskWithThreadLocal implements Runnable{
private Map<Object,Object> threadLocalMap;
private Runnable delegate;
public TaskWithThreadLocal(Runnable delegate, Map<Object,Object> threadLocalMap){
this.delegate =delegate;
this.threadLocalMap =threadLocalMap;
}
/**
* 重写run方法，在执行run方法前设置线程变量，执行run方法后清除线程变量
* 同时，打印了运行时的异常信息，并吞掉了delegate.run()运行时的异常，不往外抛
* （线程池默认会在任务运行异常后抛出异常，并销毁掉线程对象本身，也就是如果每个任务都运行异常了，那么用线程池的效率还不如直接新建线程，详情见ThreadPoolExecutor类1123行runWorkers方法 ）
* jdk线程池这样处理的意义应该是希望通过将异常抛出，将异常交给线程对象本身自带的异常处理拦截器或JVM默认的全局异常处理拦截器捕获并处理，
* 这里直接去调用拦截器处理，不往外抛异常，避免线程实例的销毁
*/
@Override
public void run() {
InheritableThreadLocal.setThreadLocalsFromMap(threadLocalMap);
try{
try{
delegate.run();
//由于callable的call()方法执行过程的异常会被它的调用上级FutureTask的run()方法中处理而使异常不往外抛，为了打印异常日志这里统一进行异常日志打印的处理
if(delegate instanceof FutureTask){
try{
((FutureTask)delegate).get();
}catch (Throwable e){
logger.error(e.getMessage(),e);
Thread.currentThread().getUncaughtExceptionHandler().uncaughtException(Thread.currentThread(),e);
}
}
}catch (Throwable e){
logger.error(e.getMessage(),e);
Thread.currentThread().getUncaughtExceptionHandler().uncaughtException(Thread.currentThread(),e);
}
}finally {
InheritableThreadLocal.removeThreadLocals();
}
}
}
}
?在Runable实例放进线程池的时候，new 一个TaskWithThreadLocal类，把线程变量放到threadLocalMap里面。实际放到线程池队列里的是TaskWithThreadLocal，TaskWithThreadLocal类的run方法执行时，会执行真正想执行的Runable实例的run方法。用TaskWithThreadLocal类传递threadLocalMap内容，有点类似jdk的InheritableThreadLocal类在Thread实例化时传递ThreadLocal.ThreadLocalMap inheritableThreadLocals。
? ? ?举个例子，比如我们用线程变量记录userId。UserIdUtil：
public class UserIdUtil {
private static final InheritableThreadLocal<String> userIdLocal = new InheritableThreadLocal<>();
public static String getUserId(){
return userIdLocal.get();
}
public static void setUserId(String userId){
userIdLocal.set(userId);
}
public static void removeUserId(){
userIdLocal.remove();
}
}
只需要把原本的ThreadLocal类改成InheritableThreadLocal类，即可在InheritableThreadLocalExecutor线程池中传递线程变量。
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