Java ThreadLocal用法实例详解

Java ThreadLocal用法实例详解

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：

目录

ThreadLocal的基本使用

ThreadLocal实现原理

源码分析(基于openjdk11)

get方法：

setInitialValue方法

getEntry方法

set方法

ThreadLocalMap的set方法

replaceStaleEntry方法

cleanSomeSlots方法

rehash方法

expungeStaleEntries方法

resize方法

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。

ThreadLocal的基本使用

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 threadLocal1 = ThreadLocal.withInitial(() -> "threadLocal1 first value");

private static ThreadLocal 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。

ThreadLocal实现原理

源码分析(基于openjdk11)

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)。

get方法：

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。

setInitialValue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http://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。

getEntry方法

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;

}

set方法

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

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方法

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

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

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

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

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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