深入学习线程局部变量ThreadLocal
ThreadLocal是线程局部变量，和普通变量的不同在于：每个线程持有这个变量的一个副本，可以独立修改（set方法）和访问（get方法）这个变量，并且线程之间不会发生冲突。
类中定义的ThreadLocal实例一般会被private static修饰，这样可以让ThreadLocal实例的状态和Thread绑定在一起。业务上，一般用ThreadLocal包装一些业务ID（user ID或事务ID）——不同的线程使用的ID是不相同的。
【1】如何使用ThreadLocal
① 包装SimpleDateFormat
SimpleDateFormat是非线程安全的，多线程操作下会出现异常。测试代码如下：
public static void main(String[] args) throws Exception {
SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
Callable<Date> task = new Callable<Date>() {
@Override
public Date call() throws Exception {
return sdf.parse("20181121");
}
};
//创建固定大小线程池
ExecutorService pool = Executors.newFixedThreadPool(10);
List<Future<Date>> results = new ArrayList<>();
for (int i = 0; i < 10; i++) {
results.add(pool.submit(task));
}
//打印结果
for (Future<Date> future : results) {
System.out.println(future.get());
}
//关闭线程池
pool.shutdown();
}
测试结果如下：
这里就可以使用ThreadLocal：
public class DateFormatThreadLocal {
private static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>(){
protected DateFormat initialValue(){
return new SimpleDateFormat("yyyyMMdd");
}
};
public static final Date convert(String source) throws ParseException{
return df.get().parse(source);
}
}
注意，这里针对每个线程只需要初始化一次SimpleDateFormat对象，其实跟在自定义线程中定义一个SimpleDateFormat成员变量，并在线程初始化的时候new这个对象，效果是一样的，只是这样看起来代码更规整。
② 自定义MyRunnable被两个线程访问
下面这个例子，我们定义了一个MyRunnable对象，这个MyRunnable对象会被线程1和线程2使用，但是通过内部的ThreadLocal变量，每个线程访问到的整数都是自己单独的一份。
package org.java.learn.concurrent.threadlocal;
public class ThreadLocalExample {
public static class MyRunnable implements Runnable {
private ThreadLocal<Integer> threadLocal =
new ThreadLocal<Integer>();
@Override
public void run() {
threadLocal.set((int) (Math.random() * 100D));
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
}
System.out.println(threadLocal.get());
}
}
public static void main(String[] args) throws InterruptedException {
MyRunnable sharedRunnableInstance = new MyRunnable();
Thread thread1 = new Thread(sharedRunnableInstance);
Thread thread2 = new Thread(sharedRunnableInstance);
thread1.start();
thread2.start();
thread1.join(); //wait for thread 1 to terminate
thread2.join(); //wait for thread 2 to terminate
}
}
【2】ThreadLocal源码分析
ThreadLocal是如何被线程使用的？原理如下图所示：Thread引用和ThreadLocal引用都在栈上，Thread引用会引用一个ThreadLocalMap对象，这个map中的key是ThreadLocal对象（使用WeakReference包装），value是业务上变量的值。
首先看java.lang.Thread中的代码：
public
class Thread implements Runnable {
//......其他源码
/* 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;
//......其他源码
Thread中的threadLocals变量指向的是一个map，这个map就是ThreadLocal.ThreadLocalMap，里面存放的是跟当前线程绑定的ThreadLocal变量。inheritableThreadLocals的作用相同，里面也是存放的ThreadLocal变量，但是存放的是从当前线程的父线程继承过来的ThreadLocal变量。
再看java.lang.ThreadLocal类，主要的成员和接口如下：
① withInitial方法
Java 8以后用于初始化ThreadLocal的一种方法，在外部调用get()方法的时候，会通过Supplier确定变量的初始值：
public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
return new SuppliedThreadLocal<>(supplier);
}
② get方法
获取当前线程的变量副本，如果当前线程还没有创建该变量的副本，则需要通过调用initialValue方法来设置初始值。
get方法的源代码如下，首先通过当前线程获取当前线程对应的map。如果map不为空，则从map中取出对应的Entry，然后取出对应的值。如果map为空，则调用setInitialValue设置初始值。如果map不为空，当前ThreadLocal实例对应的Entry为空，则也需要设置初始值。
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
其中Entry是ThreadLocal静态内部类ThreadLocalMap的静态内部类：
static class ThreadLocalMap {
/**
* The entries in this hash map extend WeakReference, using
* its main ref field as the key (which is always a
* ThreadLocal object). Note that null keys (i.e. entry.get()
* == null) mean that the key is no longer referenced, so the
* entry can be expunged from table. Such entries are referred to
* as "stale entries" in the code that follows.
*/
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
//...
}
③ set方法
set方法跟get方法一样，先获取当前线程对应的map。如果map为空，则调用createMap创建map，否则将变量的值放入map——key为当前这个ThreadLocal对象，value为变量的值。
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
createMap方法如下：
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
④ remove方法，删除当前线程绑定的这个副本
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
}
ThreadLocalMap.remove方法如下：
/**
* Remove the entry for key.
*/
private void remove(ThreadLocal<?> key) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
if (e.get() == key) {
e.clear();
expungeStaleEntry(i);
return;
}
}
}
⑤ ThreadLocalMap构造函数
ThreadLocalMap中有一个Entry[] table，其中维护了一系列的entry。table初始化容量为16。
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
⑥ private static final int HASH_INCREMENT = 0x61c88647;
这个值是HASH_INCREMENT的值，普通的hashmap是使用链表来处理冲突的，但是ThreadLocalMap是使用线性探测法来处理冲突的，HASH_INCREMENT就是每次增加的步长，根据参考资料所说，选择这个数字是为了让冲突概率最小。
/**
* The difference between successively generated hash codes - turns
* implicit sequential thread-local IDs into near-optimally spread
* multiplicative hash values for power-of-two-sized tables.
*/
private static final int HASH_INCREMENT = 0x61c88647;
【3】父子进程数据共享
InheritableThreadLocal主要用于子线程创建时，需要自动继承父线程的ThreadLocal变量，实现子线程访问父线程的threadlocal变量。InheritableThreadLocal继承了ThreadLocal，并重写了childValue、getMap、createMap三个方法。
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
*/
//**
* 创建线程的时候，如果需要继承且父线程中Thread-Local变量，则需要将父线程中的ThreadLocal变量一次拷贝过来。
*/
protected T childValue(T parentValue) {
return parentValue;
}
/**
* 由于重写了getMap，所以在操作InheritableThreadLocal变量的时候，将只操作Thread类中的inheritableThreadLocals变量，与threadLocals变量没有关系
**/
ThreadLocalMap getMap(Thread t) {
return t.inheritableThreadLocals;
}
/**
* 跟getMap类似，set或getInheritableThreadLocal变量的时候，将只操作Thread类中的inheritableThreadLocals变量
*/
void createMap(Thread t, T firstValue) {
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}
}
关于childValue多说两句，拷贝是如何发生的？首先看Thread.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");
}
this.name = name.toCharArray();
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();
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();
}
然后看ThreadLocal.createInheritedMap方法，最终会调用到newThreadLocalMap方法，这里InheritableThreadLocal对childValue做了重写，可以看出，这里确实是将父线程关联的ThreadLocalMap中的内容依次拷贝到子线程的ThreadLocalMap中了。
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
/**
* Construct a new map including all Inheritable ThreadLocals
* from given parent map. Called only by createInheritedMap.
*
* @param parentMap the map associated with parent thread.
*/
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++;
}
}
}
}
【4】ThreadLocal垃圾回收和实际应用
① ThreadLocal对象何时被回收？
ThreadLocalMap中的key是ThreadLocal对象，然后ThreadLocal对象时被WeakReference包装的，这样当没有强引用指向该ThreadLocal对象之后，或者说Map中的ThreadLocal对象被判定为弱引用可达时，就会在垃圾收集中被回收掉。看下Entry的定义：
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
② ThreadLocal和线程池一起使用会有哪些问题？
ThreadLocal对象的生命周期跟线程的生命周期一样长，那么如果将ThreadLocal对象和线程池一起使用，就可能会遇到这种情况：一个线程的ThreadLocal对象会和其他线程的ThreadLocal对象串掉，一般不建议将两者一起使用。
③ Dubbo中对ThreadLocal的使用
从Dubbo中找到了ThreadLocal的例子，它主要是用在请求缓存的场景，具体代码如下：
@Activate(group = {Constants.CONSUMER, Constants.PROVIDER}, value = Constants.CACHE_KEY)
public class CacheFilter implements Filter {
private CacheFactory cacheFactory;
public void setCacheFactory(CacheFactory cacheFactory) {
this.cacheFactory = cacheFactory;
}
@Override
public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
if (cacheFactory != null && ConfigUtils.isNotEmpty(invoker.getUrl().getMethodParameter(invocation.getMethodName(), Constants.CACHE_KEY))) {
Cache cache = cacheFactory.getCache(invoker.getUrl(), invocation);
if (cache != null) {
String key = StringUtils.toArgumentString(invocation.getArguments());
Object value = cache.get(key);
if (value != null) {
if (value instanceof ValueWrapper) {
return new RpcResult(((ValueWrapper)value).get());
} else {
return new RpcResult(value);
}
}
Result result = invoker.invoke(invocation);
if (!result.hasException()) {
cache.put(key, new ValueWrapper(result.getValue()));
}
return result;
}
}
return invoker.invoke(invocation);
}
可以看出，在RPC调用（invoke）的链路上，会先使用请求参数判断当前线程是否刚刚发起过同样参数的调用——这个调用会使用ThreadLocalCache保存起来。具体的看ThreadLocalCache的实现如下：
package org.apache.dubbo.cache.support.threadlocal;
import org.apache.dubbo.cache.Cache;
import org.apache.dubbo.common.URL;
import java.util.HashMap;
import java.util.Map;
/**
* ThreadLocalCache
*/
public class ThreadLocalCache implements Cache {
//ThreadLocal里存放的是参数到结果的映射
private final ThreadLocal<Map<Object, Object>> store;
public ThreadLocalCache(URL url) {
this.store = new ThreadLocal<Map<Object, Object>>() {
@Override
protected Map<Object, Object> initialValue() {
return new HashMap<Object, Object>();
}
};
}
@Override
public void put(Object key, Object value) {
store.get().put(key, value);
}
@Override
public Object get(Object key) {
return store.get().get(key);
}
}
④ RocketMQ中ThreadLocal使用
在RocketMQ中，也找到了ThreadLocal的身影，它是用在消息发送的场景。MQClientAPIImpl是RMQ中负责将消息发送到服务端的实现，其中有一个步骤需要选择一个具体的队列。选择具体的队列的时候，不同的线程有自己负责的index值，这里使用了ThreadLocal的机制。
可以看下ThreadLocalIndex的实现：
package org.apache.rocketmq.client.common;
import java.util.Random;
public class ThreadLocalIndex {
private final ThreadLocal<Integer> threadLocalIndex = new ThreadLocal<Integer>();
private final Random random = new Random();
public int getAndIncrement() {
Integer index = this.threadLocalIndex.get();
if (null == index) {
index = Math.abs(random.nextInt());
if (index < 0)
index = 0;
this.threadLocalIndex.set(index);
}
index = Math.abs(index + 1);
if (index < 0)
index = 0;
this.threadLocalIndex.set(index);
return index;
}
@Override
public String toString() {
return "ThreadLocalIndex{" +
"threadLocalIndex=" + threadLocalIndex.get() +
'}';
}
}