本文实例讲述了Java ThreadLocal用法。分享给大家供大家参考，具体如下：
ThreadLocal实现了Java中线程局部变量。所谓线程局部变量就是保存在每个线程中独有的一些数据，我们知道一个进程中的所有线程是共享该进程的资源的，线程对进程中的资源进行修改会反应到该进程中的其他线程上，如果我们希望一个线程对资源的修改不会影响到其他线程，那么就需要将该资源设为线程局部变量的形式。
如下示例所示，定义两个ThreadLocal变量，然后分别在主线程和子线程中对线程局部变量进行修改，然后分别获取线程局部变量的值：
public class ThreadLocalTest {
private static ThreadLocal<String> threadLocal1 = ThreadLocal.withInitial(() -> "threadLocal1 first value");
private static ThreadLocal<String> threadLocal2 = ThreadLocal.withInitial(() -> "threadLocal2 first value");
public static void main(String[] args) throws Exception{
Thread thread = new Thread(() -> {
System.out.println("================" + Thread.currentThread().getName() + " enter=================");
// 子线程中打印出初始值
printThreadLocalInfo();
// 子线程中设置新值
threadLocal1.set("new thread threadLocal1 value");
threadLocal2.set("new thread threadLocal2 value");
// 子线程打印出新值
printThreadLocalInfo();
System.out.println("================" + Thread.currentThread().getName() + " exit=================");
});
thread.start();
// 等待新线程执行
thread.join();
// 在main线程打印threadLocal1和threadLocal2，验证子线程对这两个变量的修改是否会影响到main线程中的这两个值
printThreadLocalInfo();
// 在main线程中给threadLocal1和threadLocal2设置新值
threadLocal1.set("main threadLocal1 value");
threadLocal2.set("main threadLocal2 value");
// 验证main线程中这两个变量是否为新值
printThreadLocalInfo();
}
private static void printThreadLocalInfo() {
System.out.println(Thread.currentThread().getName() + ": " + threadLocal1.get());
System.out.println(Thread.currentThread().getName() + ": " + threadLocal2.get());
}
}
运行结果如下：
================Thread-0 enter================= Thread-0: threadLocal1 first value Thread-0: threadLocal2 first value Thread-0: new thread threadLocal1 value Thread-0: new thread threadLocal2 value ================Thread-0 exit================= main: threadLocal1 first value main: threadLocal2 first value main: main threadLocal1 value main: main threadLocal2 value
如果子线程对threadLocal1和threadLocal2的修改会影响到main线程中的threadLocal1和threadLocal2，那么在main线程第一次printThreadLocalInfo();打印出的应该是修改后的新值，即为new thread threadLocal1 value和new thread threadLocal2 value和，但实际打印结果并不是这样，说明在新线程中对threadLocal1和threadLocal2的修改并不会影响到main线程中的这两个变量，似乎main线程中的threadLocal1和threadLocal2作用域仅局限于main线程，新线程中的threadLocal1和threadLocal2作用域仅局限于新线程，这就是线程局部变量的由来。
如下图所示
每个线程对象里会持有一个java.lang.ThreadLocal.ThreadLocalMap类型的threadLocals成员变量，而ThreadLocalMap里有一个java.lang.ThreadLocal.ThreadLocalMap.Entry[]类型的table成员，这是一个数组，数组元素是Entry类型，Entry中相当于有一个key和value，key指向所有线程共享的java.lang.ThreadLocal对象，value指向各线程私有的变量，这样保证了线程局部变量的隔离性，每个线程只是读取和修改自己所持有的那个value对象，相互之间没有影响。
源码包括ThreadLocal和ThreadLocalMap，ThreadLocalMap是ThreadLocal内定义的一个静态内部类，用于存储实际的数据。当调用ThreadLocal的get或者set方法时都有可能创建当前线程的threadLocals成员(ThreadLocalMap类型)。
ThreadLocal的get方法定义如下
/**
* 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() {
// 获取当前线程
Thread t = Thread.currentThread();
// 获取当前线程的threadLocals成员变量，这是一个ThreadLocalMap
ThreadLocalMap map = getMap(t);
// threadLocals不为null则直接从threadLocals中取出ThreadLocal
// 对象对应的值
if (map != null) {
// 从map中获取当前ThreadLocal对象对应Entry对象
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
// 获取ThreadLocal对象对应的value值
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
// threadLocals为null，则需要创建ThreadLocalMap对象并赋给
// threadLocals，将当前ThreadLocal对象作为key，调用initialValue
// 获得的初始值作为value，放置到threadLocals的entry中;
// 或者threadLocals不为null，但在threadLocals中未
// 找到当前ThreadLocal对象对应的entry，则需要向threadLocals添加新的
// entry，该entry以当前的ThreadLocal对象作为key，调用initialValue
// 获得的值作为value
return setInitialValue();
}
/**
* Get the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @return the map
*/
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
当Thread的threadLocals为null，或者在Thread的threadLocals中未找到当前ThreadLocal对象对应的entry，则进入到setInitialValue方法；否则进入到ThreadLocalMap的getEntry方法。
定义如下：
private T setInitialValue() {
// 获取初始值,如果我们在定义ThreadLocal对象时实现了ThreadLocal
// 的initialValue方法，就会调用我们自定义的方法来获取初始值，否则
// 使用initialValue的默认实现返回null值
T value = initialValue();
Thread t = Thread.currentThread();
// 获取当前线程的threadLocals成员
ThreadLocalMap map = getMap(t);
if (map != null) {
// 若threadLocals存在则将ThreadLocal对象对应的value设置为初始值
map.set(this, value);
} else {
// 否则创建threadLocals对象并设置初始值
createMap(t, value);
}
if (this instanceof TerminatingThreadLocal) {
TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
}
return value;
}
createMap方法实现
/**
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
*/
void createMap(Thread t, T firstValue) {
// 创建一个ThreadLocalMap对象，用当前ThreadLocal对象和初始值value来
// 构造ThreadLocalMap中table的第一个entry。ThreadLocalMap对象赋
// 给线程的threadLocals成员
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
ThreadLocalMap的构造方法定义如下：
/**
* Construct a new map initially containing (firstKey, firstValue).
* ThreadLocalMaps are constructed lazily, so we only create
* one when we have at least one entry to put in it.
*/
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
// 构造table数组，数组大小为INITIAL_CAPACITY
table = new Entry[INITIAL_CAPACITY];
// 计算key(ThreadLocal对象)在table中的索引
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
// 用ThreadLocal对象和value来构造entry对象，并放到table的第i个位置
table[i] = new Entry(firstKey, firstValue);
size = 1;
// 设置table的阈值，当table中元素个数超过该阈值时需要对table
// 进行resize，通常在调用ThreadLocalMap的set方法时会发生resize
setThreshold(INITIAL_CAPACITY);
}
/**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
这里firstKey.threadLocalHashCode是ThreadLocal中定义的一个hashcode，使用该hashcode进行hash运算从而找到该ThreadLocal对象对应的entry在table中的索引。
定义如下：
/**
* 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) {
// 根据ThreadLocal的hashcode计算该ThreadLocal对象在table中的位置
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
// e为null则table不存在key对应的entry;
// e.get() != key 可能是由于hash冲突导致key对应的entry在table
// 的另外一个位置，需要继续查找
if (e != null && e.get() == key)
return e;
else
// e==null或者e.get() != key 继续查找key对应的entry
return getEntryAfterMiss(key, i, e);
}
getEntryAfterMiss方法定义如下：
/**
* 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;
// 从table的第i个位置一直往后找，直到找到键为key的entry为止
while (e != null) {
ThreadLocal<?> k = e.get();
// 若k==key，则找到了entry
if (k == key)
return e;
// k == null 需要删除该entry
if (k == null)
expungeStaleEntry(i);
// k != key && k != null 继续往后寻找，nextIndex就是取(i+1)
// 即table中第(i+1)个位置的entry
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
expungeStaleEntry方法删除key为null的entry，删除后对staleSlot位置的entry和其后第一个为null的entry之间的entry进行一个rehash操作，rehash的目的是降低table发生碰撞的概率:
/**
* 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
// 删除staleSlot位置的entry
tab[staleSlot].value = null;
tab[staleSlot] = null;
// table中元素个数减一
size--;
// Rehash until we encounter null
// 将table中staleSlot处entry和下一个为null的entry之间的
// entry重新进行hash放置到新的位置
// 遇到的entry的key为null则删除该entry
Entry e;
int i;
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
// e是下一个entry
ThreadLocal<?> k = e.get();
if (k == null) {
// 若entry的key为null，则删除
e.value = null;
tab[i] = null;
size--;
} else {
// entry的key不为null，需要将entry放到新的位置
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.
// tab[h]不为null则发生冲突，继续寻找下一个位置
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
ThreadLocal的set方法定义如下：
/**
* 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();
// 获取当前线程的threadLocals
ThreadLocalMap map = getMap(t);
// threadLocals不为null直接设置新值
if (map != null) {
map.set(this, value);
} else {
// threadLocals为null则需要创建ThreadLocalMap对象并赋给
// Thread的threadLocals成员
createMap(t, value);
}
}
createMap前面已经分析过，接下来分析ThreadLocalMap的set方法
ThreadLocalMap的set方法定义如下，将当前的ThreadLocal对象作为key，传入的value为值，用key和value创建entry，放到table中适当的位置：
/**
* 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;
// 用key计算entry在table中的位置
int i = key.threadLocalHashCode & (len-1);
// tab[i]不为null的话，则第i个位置已经存在有效的entry，需要继续
// 往后寻找新的位置
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
// 找到与key相同的entry，直接更新value的值
if (k == key) {
e.value = value;
return;
}
// 遇到key为null的entry，删除该entry
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
// 此时第i个位置entry为null，将新entry放置到这个位置
tab[i] = new Entry(key, value);
int sz = ++size;
// 试图清除无效的entry，若清除失败并且table中有效entry个数
// 大于threshold，这进行rehash操作
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
replaceStaleEntry的作用是用set方法传过来的key和value构造entry，将这个entry放到staleSlot后面的某个位置：
/**
* 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).
// 从staleSlot往前找到第一个key为null的entry的位置
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
// 从staleSlot位置往后寻找
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.
// 若k与key相同，则直接更新value
if (k == key) {
e.value = value;
// 将原来staleSlot位置的entry放置到第i个位置，此时tab[i]处的entry的key为null
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
// 从staleSlot处往前未找到key为null的entry
if (slotToExpunge == staleSlot)
// tab[i]处entry的key为null，也即tab[slotToExpunge]处entry的key为null
slotToExpunge = i;
// 清除slotToExpunge位置的entry并进行rehash操作.....
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);
}
以下源码只可意会，不可言传…不再做说明
cleanSomeSlots方法：
/**
* 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;
}
rehash方法：
/**
* Re-pack and/or re-size the table. First scan the entire
* table removing stale entries. If this doesn't sufficiently
* shrink the size of the table, double the table size.
*/
private void rehash() {
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
}
expungeStaleEntries方法：
/**
* Expunge all stale entries in the table.
*/
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);
}
}
resize方法：
/**
* Double the capacity of the table.
*/
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0;
for (Entry e : oldTab) {
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;
}
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