ThreadLocal 源码分析
2018-04-12 本文已影响14人
Sotardust
写在前面
我们知道在使用handler进行消息传递时，需要创建Looper.prepare(),以及执行Looper.loop()方法
不了解的可以看下Handler消息机制源码分析。
查看Looper.java$prepare方法源码
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
//创建MessageQueue 这也是为什么一个Looper中只有一个MessageQueue的原因
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
可以看到是sThreadLocal调用了set方法创建Looper ，查看Looper.java代码发现Looper是存储在ThreadLocal里面的
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
ThreadLocal是什么呢 ，ThreadLocal是存储当前线程数据的数据存储类。
我们知道Looper.loop()方法中调用myLooper()获取Looper对象，查看myLooper方法源码
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
发现是直接通过调用ThreadLocal类中的get方法获取存储在ThreadLocal中的Looper。
源码分析
这里先分析ThreadLocal的set方法查看其源码
public void set(T value) {
//获取当前线程
Thread t = Thread.currentThread();
// ThreadLocalMap为ThreadLocal的静态内部类，后面会对该类进行分析
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
先看ThreadLocal类的createMap方法
void createMap(Thread t, T firstValue) {
//t.threadLocals是一个ThreadLocalMap对象
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
//查看Thread类
ThreadLocal.ThreadLocalMap threadLocals = null;
每个线程通过ThreadLocal.ThreadLocalMap与ThreadLocal进行绑定，确保每个线程访问到的thread local variable都是该线程的。
现在分析ThreadLocal类的get方法
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
}
先看setInitialValue()方法源码
private T setInitialValue() {
//为thread local 初始化值
T value = initialValue();
//下面则与ThreadLocal的set方法源码相同
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
从ThreadLocal的get方法中可以看到该方法调用了ThreadLocalMap中的set方法和getEntry方法，接下来分析ThreadLocalMap的实现。
我们先看ThreadLocalMap的介绍：是一个自定义的哈希映射表用于维护线程本地值。查看ThreadLocalMap的数据结构：
//Entry 为ThreadLocalMap的静态内部类，继承弱引用的ThreadLocal
static class Entry extends WeakReference<ThreadLocal> {
// 实际保存的对象值
Object value;
Entry(ThreadLocal k, Object v) {
super(k);
value = v;
}
}
//Entry[] 的初始大小，其值必须是2的n次幂
private static final int INITIAL_CAPACITY = 16;
//实际存放对象的容器，其大小必须是2的n次幂
private Entry[] table;
//Entry表元素的数量
private int size = 0;
//哈希表的扩容阈值默认值为0
private int threshold; // Default to 0
查看ThreadLocalMap构造函数
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
//用firstKey的threadLocalHashCode与初始值取模获取元素放置的下标位置
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
//初始化该节点
table[i] = new Entry(firstKey, firstValue);
size = 1;
//设置阈值
setThreshold(INITIAL_CAPACITY);
}
firstKey 为ThreadLocal对象，ThreadLocalHashCode在被ThreadLocal创建的时候就生成了
// 相当于ThreadLocal的ID
private final int threadLocalHashCode = nextHashCode();
接下来查看ThreadLocalMap类中的getEntry方法源码
private Entry getEntry(ThreadLocal<?> key) {
//根据key获取索引
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
//如果对应的Entry存在且未失效则返回
if (e != null && e.get() == key)
return e;
else
//使用线性探测，继续查找目标Entry
return getEntryAfterMiss(key, i, e);
}
查看getEntryAfterMiss方法源码
private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
// 基于线性探测法不断向后探测直到遇到空entry。
while (e != null) {
ThreadLocal<?> k = e.get();
if (k == key)
return e;
if (k == null)
//k为空则说明Entry中对应的ThreadLocal已经被回收，调用该方法来清理无效的entry
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
查看expungeStaleEntry方法源码
// 该函数是ThreadLocal中的核心清理函数，从staleSlot开始遍历
//将对应Entry中的Vaule值置为空
private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;
int len = tab.length;
// 将staleSlot对应的Entry已经Entry中的Value置为null
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
// Rehash until we encounter null
Entry e;
int i;
//从staleSlot开始遍历
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
// 清理对应ThreadLocal已经被回收的entry
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.
// 从计算出来的哈希位开始往后查找，找到一个适合它的空位
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
查看ThreadLocalMap的set方法源码
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);
//线性探测
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
//通过key查找到对应的Entry
if (k == key) {
e.value = value;
return;
}
//替换失效的Entry
if (k == null) {
//如果entry里对应的key为null的话，表明此entry为 旧的，就将其替换为当前的key和value
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
//清除失效entry并进行扩容
rehash();
}
接着查看replaceStaleEntry方法源码
private void replaceStaleEntry(ThreadLocal<?> key, Object value,
int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
int slotToExpunge = staleSlot;
//向前扫描，查找最前的一个无效slot
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
//向后遍历table
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
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;
// 清理无效slot
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
// 如果当前的slot已经无效，并且向前扫描过程中没有无效slot，
// 则更新slotToExpunge为当前位置
if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
// key没找到就创建一个新的entry
tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);
// 如果运行中有其他无效的slot则删除它们对应的entry
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
查看cleanSomeSlots方法源码
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;
}
清除无效的slot发现最后还是调用的expungeStaleEntry方法。
查看ThreadLocalMap的rehash方法源码
private void rehash() {
//清除table中所有的失效的Entry
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
//对table的容量进行2倍扩容
resize();
}
//继续查看
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
//以原来表容量的2倍进行扩容
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;
}
分析结束。
