JavaThreadLocal用法的实例分析(JavaThreadLocal,编程语言)

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 valueThread-0: threadLocal2 first valueThread-0: new thread threadLocal1 valueThread-0: new thread threadLocal2 value================Thread-0 exit=================main: threadLocal1 first valuemain: threadLocal2 first valuemain: main threadLocal1 valuemain: 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;

}adLocal