ThreadLocal
记录一套比较有参考价值的：
并发容器之ThreadLocal
一篇文章，从源码深入详解ThreadLocal内存泄漏问题
设计目的
官方的解释如下：
This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its {@code get} or {@code set} method) has its own, independently initialized copy of the variable. {@code ThreadLocal} instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID).
大致翻译
该类提供了线程局部 (thread-local) 变量。这些变量不同于它们的普通对应物，因为访问某个变量（通过其 get 或 set 方法）的每个线程都有自己的局部变量，它独立于变量的初始化副本。ThreadLocal 实例通常是类中的 private static 字段，它们希望将状态与某一个线程（例如，用户 ID 或事务 ID）相关联。
官方示例：
import java.util.concurrent.atomic.AtomicInteger;
public class ThreadId {
// Atomic integer containing the next thread ID to be assigned
private static final AtomicInteger nextId = new AtomicInteger(0);
// Thread local variable containing each thread's ID
private static final ThreadLocal threadId =
new ThreadLocal() {
@Override protected Integer initialValue() {
return nextId.getAndIncrement();
}
};
// Returns the current thread's unique ID, assigning it if necessary
public static int get() {
return threadId.get();
}
} 从官方的说明和例子中可以看出，这个ThreadLocal是为了给当前线程创建私有化变量副本的。他创建堆区中的对象（可以被所有线程共享）的副本到Thread Local中给当前线程使用。也就是只要请求是一个新的线程，那么他在这个请求的任何地方拿目标对象都是该线程私有的，因为事实上只是以某个对象为原型new了一个出来。总之他的意义在于线程隔离。
和Thread、ThreadLocalMap关系
ThreadLocal 和 ThreadLocalMap
**ThreadLocalMap是ThreadLocal的静态内部类，其内部还有一个继承了WeakReference
的Entry的内部类
下面是官方源码说明：**
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.
一是这个Entry的实现是继承了WeakReference（弱引用）的，再一个他的key就是ThreadLocal本身。
ThreadLocalMap 和 Thread
ThreadLocalMap是Thread的属性，正因如此，ThreadLocal才得以实现。
/*
* 当前线程的ThreadLocalMap，主要存储该线程自身的ThreadLocal
*/
ThreadLocal.ThreadLocalMap threadLocals = null;
/*
* InheritableThreadLocal，自父线程集成而来的ThreadLocalMap，
* 主要用于父子线程间ThreadLocal变量的传递
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;源码分析
**源码中主要的方法有
set、get、initialValue、withInitial、setInitialValue、remove，下面一个一个分析**
initialValue()
该方法由protect修饰，显然是为了重写。
**该段说明大意：该方法返回当前线程的ThreadLocal变量的“初始值”。除非在此之前已经调用过set()方法设置值，否则这个方法会在第一次调用get()获取变量时调用。正常情况下这个方法只会在第一次调用get()后调用一次，除非在get()后使用过remove()方法。
这个实现返回了null，如果有需要初始值不是null的话，就需要重写该方法，通常使用匿名内部类的方式。**
/**
* Returns the current thread's "initial value" for this
* thread-local variable. This method will be invoked the first
* time a thread accesses the variable with the {@link #get}
* method, unless the thread previously invoked the {@link #set}
* method, in which case the {@code initialValue} method will not
* be invoked for the thread. Normally, this method is invoked at
* most once per thread, but it may be invoked again in case of
* subsequent invocations of {@link #remove} followed by {@link #get}.
*
* This implementation simply returns {@code null}; if the
* programmer desires thread-local variables to have an initial
* value other than {@code null}, {@code ThreadLocal} must be
* subclassed, and this method overridden. Typically, an
* anonymous inner class will be used.
*
* @return the initial value for this thread-local
*/
protected T initialValue() {
return null;
}
get()
该方法用于获取ThreadLocal中的对象，首先拿到当前线程的线程对象，通过当前线程对象获取对应的ThreadLocalMap。判断map是否为空，然后根据当前的ThreadLocal对象查找对应对象，如果不为空就返回，否则调用setInitialValue()
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
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();
}setInitialValue()
当get()时没有值的话，则会调用该方法，该方法会调用initialValue()设置初始值。并且从这里可以看出，其实在存储的时候不同的是Map，而不是map中的key。在取值的时候是使用象同的key在各个线程中拿到不同的map从而拿到不同的value。
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}### set()
该方法用于给当前线程的LocalThread变量设置值，大多数子类不需要使用这个方法，只依赖于initialValue()方法来设置线程局部变量的值。
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}### remove()
移除变量，如果在调用这个方法后再次调用get()，那么会再次执行initialValue()。
/**
* Removes the current thread's value for this thread-local
* variable. If this thread-local variable is subsequently
* {@linkplain #get read} by the current thread, its value will be
* reinitialized by invoking its {@link #initialValue} method,
* unless its value is {@linkplain #set set} by the current thread
* in the interim. This may result in multiple invocations of the
* {@code initialValue} method in the current thread.
*
* @since 1.5
*/
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
}withInitial()
该方法java8引入，为了可以使用lambda来设置初始值，如：
**private static ThreadLocalthreadLocal =
ThreadLocal.withInitial(() ->newCpc0010Manager());**
/**
* Creates a thread local variable. The initial value of the variable is
* determined by invoking the {@code get} method on the {@code Supplier}.
*
* @param the type of the thread local's value
* @param supplier the supplier to be used to determine the initial value
* @return a new thread local variable
* @throws NullPointerException if the specified supplier is null
* @since 1.8
*/
public static ThreadLocal withInitial(Supplier supplier) {
return new SuppliedThreadLocal<>(supplier);
}可能存在的内存泄露
从下图可以看到，ThreadLocalMap的key对ThreadLocal使用了弱引用，当ThreadLocal成为null以后Map中的key会被回收，但是该Map中的Entry因为有强引用则不会被回收，则造成了key为null的Entry的存在，从而造成内存泄露。因此源码中为了避免这种情况在set()、get()、remove()时都会清除key为null的Entry。
**使用时应避免ThreadLocal和线程池一起使用，不仅容易OOM而且会造成脏读。
更多可参见：**
get()路线
这里是一个正常的get()方法，当ThreadLocalMap不为空的时候则调用getEntry(this)
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();
}
getEntry()方法如下，这是一个快速命中的策略，直接命中现有key，如果没有命中就会调用getEntryAfterMiss()，这样设计的目的是为了最大限度的提高性能。
/**
* Get the entry associated with key. This method
* itself handles only the fast path: a direct hit of existing
* key. It otherwise relays to getEntryAfterMiss. This is
* designed to maximize performance for direct hits, in part
* by making this method readily inlinable.
*
* @param key the thread local object
* @return the entry associated with key, or null if no such
*/
private Entry getEntry(ThreadLocal key) {
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}
getEntryAfterMiss()是当目标key的hash值没有直接命中而进行后续处理的方法。当key没有命中时会以当前下标开始遍历Entry，并使用expungeStaleEntry()把遇到的key为null的给清除。
/**
* Version of getEntry method for use when key is not found in
* its direct hash slot.
*
* @param key the thread local object
* @param i the table index for key's hash code
* @param e the entry at table[i]
* @return the entry associated with key, or null if no such
*/
private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
while (e != null) {
ThreadLocal k = e.get();
if (k == key)
return e;
if (k == null)
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
该方法就是将当前key为null的Entry和value也设为null，然后从当前index向后遍历，将key为null的Entry和value也设为null，以便GC。方法返回下一个Entery为null的下标。
/**
* Expunge a stale entry by rehashing any possibly colliding entries
* lying between staleSlot and the next null slot. This also expunges
* any other stale entries encountered before the trailing null. See
* Knuth, Section 6.4
*
* @param staleSlot index of slot known to have null key
* @return the index of the next null slot after staleSlot
* (all between staleSlot and this slot will have been checked
* for expunging).
*/
private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
// Rehash until we encounter null
//向后遍历，直到Entry为null
Entry e;
int i;
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal k = e.get();
//如果key为null，就设置该Entery和value为null
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
//处理hash重复情况
int h = k.threadLocalHashCode & (len - 1);
//如果不是，则设置当前index的Entry为null
if (h != i) {
tab[i] = null;
// Unlike Knuth 6.4 Algorithm R, we must scan until
// null because multiple entries could have been stale.
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}set()路线
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}调用map.set()，遍历Map，如果key已经存在则替换，在遍历的同时发现key为null则调用replaceStaleEntry()。否则就新建然后存入，存入时调用cleanSomeSlots()，去除旧Entry。
/**
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
private void set(ThreadLocal key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
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)]) {
ThreadLocal k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}replaceStaleEntry()方法用于替换陈旧的Entery。
/**
* Replace a stale entry encountered during a set operation
* with an entry for the specified key. The value passed in
* the value parameter is stored in the entry, whether or not
* an entry already exists for the specified key.
*
* As a side effect, this method expunges all stale entries in the
* "run" containing the stale entry. (A run is a sequence of entries
* between two null slots.)
*
* @param key the key
* @param value the value to be associated with key
* @param staleSlot index of the first stale entry encountered while
* searching for key.
*/
private void replaceStaleEntry(ThreadLocal key, Object value,
int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
// Back up to check for prior stale entry in current run.
// We clean out whole runs at a time to avoid continual
// incremental rehashing due to garbage collector freeing
// up refs in bunches (i.e., whenever the collector runs).
int slotToExpunge = staleSlot;
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
// Find either the key or trailing null slot of run, whichever
// occurs first
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal k = e.get();
// If we find key, then we need to swap it
// with the stale entry to maintain hash table order.
// The newly stale slot, or any other stale slot
// encountered above it, can then be sent to expungeStaleEntry
// to remove or rehash all of the other entries in run.
if (k == key) {
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
if (slotToExpunge == staleSlot)
slotToExpunge = i;
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
// If we didn't find stale entry on backward scan, the
// first stale entry seen while scanning for key is the
// first still present in the run.
if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
// If key not found, put new entry in stale slot
tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);
// If there are any other stale entries in run, expunge them
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}cleanSomeSlots()方法，主要用于控制清除“脏Entry”的执行，他会现在hash表范围内搜索是否有“脏Entry”，如果有的话就调用expungeStaleEntry()在全表范围内进一步扫描清理。这个方法的设计是为了提高效率。
/**
* Heuristically scan some cells looking for stale entries.
* This is invoked when either a new element is added, or
* another stale one has been expunged. It performs a
* logarithmic number of scans, as a balance between no
* scanning (fast but retains garbage) and a number of scans
* proportional to number of elements, that would find all
* garbage but would cause some insertions to take O(n) time.
*
* @param i a position known NOT to hold a stale entry. The
* scan starts at the element after i.
*
* @param n scan control: {@code log2(n)} cells are scanned,
* unless a stale entry is found, in which case
* {@code log2(table.length)-1} additional cells are scanned.
* When called from insertions, this parameter is the number
* of elements, but when from replaceStaleEntry, it is the
* table length. (Note: all this could be changed to be either
* more or less aggressive by weighting n instead of just
* using straight log n. But this version is simple, fast, and
* seems to work well.)
*
* @return true if any stale entries have been removed.
*/
private boolean cleanSomeSlots(int i, int n) {
boolean removed = false;
Entry[] tab = table;
int len = tab.length;
do {
i = nextIndex(i, len);
Entry e = tab[i];
if (e != null && e.get() == null) {
n = len;
removed = true;
i = expungeStaleEntry(i);
}
} while ( (n >>>= 1) != 0);
return removed;
}测试Demo
共享类
待共享的bean为Cpc0010Manager，则在该类中添加私有静态变量ThreadLocal，并重写initialValue()方法设置初始值，添加通用的get()，set()方法。
public class Cpc0010Manager {
int id;
String name;
private static ThreadLocal threadLocal = new ThreadLocal(){
@Override
protected Cpc0010Manager initialValue(){
Cpc0010Manager cpc0010Manager=new Cpc0010Manager();
cpc0010Manager.id=0;
cpc0010Manager.name="name";
return cpc0010Manager;
}
};
public int getId() {
return id;
}
public Cpc0010Manager setId(int id) {
this.id = id;
return this;
}
public String getName() {
return name;
}
public Cpc0010Manager setName(String name) {
this.name = name;
return this;
}
public Cpc0010Manager getThreadLocal() {
return threadLocal.get();
}
public void setThreadLocal(Cpc0010Manager cpc0010Manager) {
threadLocal.set(cpc0010Manager);
}
} 线程类
添加构造方法，接受Cpc0010Manager的实例，并在run方法中调用。打印的时候分别打印传入Cpc0010Manager的属性和ThreadLocal中的属性。
public class Cpc0010Thread implements Runnable {
Cpc0010Manager cpc0010Manager;
public Cpc0010Thread(Cpc0010Manager manager) {
cpc0010Manager = manager;
}
@Override
public void run() {
System.out.println(Thread.currentThread().toString() + ":" + cpc0010Manager.getThreadLocal() + "---" + cpc0010Manager.getId() + cpc0010Manager.getThreadLocal().getName());
}
}测试类
测试时分别改变id和name观察变化。sleep是为了保证程序的先后顺序。
public static void main(String[] args) throws InterruptedException {
Cpc0010Manager cpc0010Manager=new Cpc0010Manager();
Cpc0010Thread t1=new Cpc0010Thread(cpc0010Manager);
Cpc0010Thread t2=new Cpc0010Thread(cpc0010Manager);
Cpc0010Thread t3=new Cpc0010Thread(cpc0010Manager);
cpc0010Manager.getThreadLocal();
new Thread(t1).start();
Thread.sleep(1000L);
cpc0010Manager.setId(2);
new Thread(t2).start();
cpc0010Manager.getThreadLocal().setName("newName");
Thread.sleep(1000L);
new Thread(t3).start();
}结果及小结
控制台输出结果如下：
TThread[Thread-1,5,main]:com.crrcdt.res.config.Cpc0010Manager@1c5bee7d---0localName
Thread[Thread-2,5,main]:com.crrcdt.res.config.Cpc0010Manager@3990d676---2localName
Thread[Thread-3,5,main]:com.crrcdt.res.config.Cpc0010Manager@236e9199---2localName
小结：传入的Cpc0010Manager对象是同一个实例，而从ThreadLocal中获取的都是本地私有的副本，他们的copy的对象的属性是从initialValue()方法而来，除了初始设置的值相同以外，并无其他关联。感悟就是ThreadLocal唯一解决的就是在各个线程中不用自己new的问题。
实际应用
hibernate中的session管理
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
/**
* 升级的MySessionFactory 线程局部模式
* @author xuliugen
*/
public class HibernateUtil {
private static SessionFactory sessionFactory = null;
// 使用线程局部模式
private static ThreadLocal threadLocal = new ThreadLocal();
/*
* 默认的构造函数
*/
private HibernateUtil() {
}
/*
* 静态的代码块
*/
static {
sessionFactory = new Configuration().configure().buildSessionFactory();
}
/*
* 获取全新的的session
*/
public static Session openSession() {
return sessionFactory.openSession();
}
/*
* 获取和线程关联的session
*/
public static Session getCurrentSession() {
Session session = threadLocal.get();
// 判断是是是否得到
if (session == null) {
session = sessionFactory.openSession();
// 把session放到 threadLocal，相当于该session已经于线程绑定
threadLocal.set(session);
}
return session;
}
} 总结
- ThreadLocal为每一个线程提供一个实例，防止互相影响
- ThreadLocal是一个操作类，真正起作用的原因在于Thread中引用了ThreadLocalMap
- ThreadLocal的initialValue()和withInitial()用于设定初始value
- ThreadLocal在调用remov()、get()、set()时会自动清除key为null的entry
- 由于ThreadLocalMap的Entry的key是弱引用，所以在下次gc时，会回收没有被引用的ThreadLocal，而value不会被回收，所以key为null的Entry不会被回收。
- ThreadLocal与线程时一起使用时，要避免内存泄漏和脏读问题
- ThreadLocalMap是ThreadLocal的静态内部类，并使用Entry自行实现了map功能
- ThreadLocalMap使用开放地址法解决hash冲突，因为经常修改
- ThreadLocalMap的初始大小为16，加载因子为2/3，hash表可用大小为16*2/3=10
- 清除“脏Entry”主要通过expungeStaleEntry()、replaceStaleEntry()、cleanSomeSlots()