【Android应用源码分析】Java多线程：线程本地变量ThreadLocal源码分析

ThreadLocal简介

线程本地变量ThreadLocal为变量在每个线程中都创建了一个副本，每个线程可以访问自己内部的副本变量,不能访问其他线程的该变量，线程之间互不影响。即变量是线程内共享的，线程间互斥的。

ThreadLocal类源码分析

本文通过jdk1.7）中的ThreaLocal类进行解析：

package java.lang;
import java.lang.ref.*;
import java.util.concurrent.atomic.AtomicInteger;

public class ThreadLocal<T> {

private final int threadLocalHashCode = nextHashCode();  //ThreadLocal实例hash值，用来区分不同实例

private static AtomicInteger nextHashCode =         //可以看作hash值的一个基值
new AtomicInteger();

private static final int HASH_INCREMENT = 0x61c88647;   //hash值每次增加量

private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
/* 返回此线程局部变量的当前线程的初始值。最多在每次访问线程来获得每个线程局部变量时调用此方法一次，即线程第一次
使用 get() 方法访问变量的时候。如果线程先于 get 方法调用 set(T) 方法，则不会在线程中再调用
initialValue 方法。 该实现只返回 null；如果程序员希望将线程局部变量初始化为 null 以外的某个值，
则必须为 ThreadLocal 创建子类，并重写此方法。通常，将使用匿名内部类。initialValue 的典型实现
将调用一个适当的构造方法，并返回新构造的对象。
返回：
返回此线程局部变量的初始值*/
protected T initialValue() {
return null;
}
//无参构造函数
public ThreadLocal() {
}
//注意：每个线程中都是有一个ThreadLocalMap对象，它属于Map类型，其中key为ThreadLocal对象，value为某个对象。
//从当前线程的ThreadLocalMap中取出 key为当前ThreadLocal对象 的value对象，其实key值与ThreadLocal的threadLocalHashCode值有关
/*    返回此线程局部变量的当前线程副本中的值。如果这是线程第一次调用该方法，则创建并初始化此副本。

返回：
此线程局部变量的当前线程的值*/
public T get() {
Thread t = Thread.currentThread();   //得到当前线程
ThreadLocalMap map = getMap(t);     //得到当前线程的ThreadLocalMap对象
if (map != null) {      //如果map不为null,
ThreadLocalMap.Entry e = map.getEntry(this);  //得到map中Entry实体对象；
if (e != null)   //如果e不为空，则取出Entry对象中的value值，然后返回
return (T)e.value;
}
return setInitialValue();  //如果map为null,则创建ThreadLocalMap对象，
//并且创建一个空的T对象放到map中，最后返回null
}

private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();  //得到当前线程
ThreadLocalMap map = getMap(t);  //得到当前线程的ThreadLocalMap对象
if (map != null)          //map不为空，则将value放到map
map.set(this, value);
else
createMap(t, value);   //否则创建map,然后将value放到map中
return value;
}
//把value放到当前线程的ThreadLocalMap对象中去，其中key值与当前ThreadLocal对象的threadLocalHashCode值有关
/*    将此线程局部变量的当前线程副本中的值设置为指定值。许多应用程序不需要这项功能，它们只依赖于
initialValue() 方法来设置线程局部变量的值。

参数：
value - 存储在此线程局部变量的当前线程副本中的值。*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
//删除当前线程的 ThreadLocalMap对象中 key为当前ThreadLocal 的Entry(包含key/value)
/*    移除此线程局部变量的值。这可能有助于减少线程局部变量的存储需求。如果再次访问此线程局部变量，那么在默认情况下
它将拥有其 initialValue。*/
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
}

//取得TheadLocalMap
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}

//创建TheadLocalMap
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}

static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}

T childValue(T parentValue) {
throw new UnsupportedOperationException();
}
//静态内部类ThreadLcoalMap
static class ThreadLocalMap {

static class Entry extends WeakReference<ThreadLocal> {
/** The value associated with this ThreadLocal. */
Object value;

Entry(ThreadLocal k, Object v) {
super(k);
value = v;
}
}

private static final int INITIAL_CAPACITY = 16;

private Entry[] table;

private int size = 0;

private int threshold; // Default to 0

private void setThreshold(int len) {
threshold = len * 2 / 3;
}

private static int nextIndex(int i, int len) {
return ((i + 1 < len) ? i + 1 : 0);
}

private static int prevIndex(int i, int len) {
return ((i - 1 >= 0) ? i - 1 : len - 1);
}

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);
}

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) {
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++;
}
}
}
}

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);
}

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;
}

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();
}

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;
}
}
}

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);
}

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
Entry e;
int i;
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal k = e.get();
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
int h = k.threadLocalHashCode & (len - 1);
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;
}

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;
}

private void rehash() {
expungeStaleEntries();

// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
}

private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0;

for (int j = 0; j < oldLen; ++j) {
Entry e = oldTab[j];
if (e != null) {
ThreadLocal k = e.get();
if (k == null) {
e.value = null; // Help the GC
} else {
int h = k.threadLocalHashCode & (newLen - 1);
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
}

setThreshold(newLen);
size = count;
table = newTab;
}

private void expungeStaleEntries() {
Entry[] tab = table;
int len = tab.length;
for (int j = 0; j < len; j++) {
Entry e = tab[j];
if (e != null && e.get() == null)
expungeStaleEntry(j);
}
}
}
}

ThreadLocal源码总结

ThreadLocal类主要提供的四个方法：

public T get() { }

public void set(T value) { }

public void remove() { }

protected T initialValue() { }

get()方法是用来获取ThreadLocal在当前线程中保存的变量副本，set()用来设置当前线程中变量的副本，remove()用来移除当前线程中变量的副本，initialValue()是一个protected方法，一般是用来在使用时进行重写的，它是一个延迟加载方法。

ThreadLocal是如何为每个线程创建变量的副本的：

　首先，在每个线程Thread内部有一个ThreadLocal.ThreadLocalMap类型的成员变量threadLocals，这个threadLocals就是用来存储实际的变量副本的，键值为当前ThreadLocal变量，value为变量副本（即T类型的变量）。

　　初始时，在Thread里面，threadLocals为空，当通过ThreadLocal变量调用get()方法或者set()方法，就会对Thread类中的threadLocals进行初始化，并且以当前ThreadLocal变量为键值，以ThreadLocal要保存的副本变量为value，存到threadLocals。

　　然后在当前线程里面，如果要使用副本变量，就可以通过get方法在threadLocals里面查找。

总结一下：

　　1）实际的通过ThreadLocal创建的副本是存储在每个线程自己的threadLocals中的；

　　2）为何threadLocals的类型ThreadLocalMap的键值为ThreadLocal对象，因为每个线程中可有多个threadLocal变量，就像上面代码中的longLocal和stringLocal；

　　3）在进行get之前，必须先set，否则会报空指针异常；如果想在get之前不需要调用set就能正常访问的话，必须重写initialValue()方法。如果没有先set的话，即在map中查找不到对应的存储，则会通过调用setInitialValue方法返回i，而在setInitialValue方法中，有一个语句是T value = initialValue()， 而默认情况下，initialValue方法返回的是null。

　　4）最常见的ThreadLocal使用场景为 用来解决 数据库连接、Session管理等。

参考文章：

https://passport.csdn.net/account/login?ref=toolbar

http://www.iteye.com/topic/103804

http://www.cnblogs.com/dolphin0520/p/3920407.html

http://blog.csdn.net/imzoer/article/details/8262101