JavaThreadLocal用法的实例分析

JavaThreadLocal用法的实例分析，针对这个问题，这篇文章详细介绍了相对应的分析和解答，希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。

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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 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。

ThreadLocal实现原理

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; 

   }adLocal

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