Java unsafe操做指北

Unsafe是Java無鎖操做的基石,在無鎖併發類中都少不了它們的身影,好比ConcurrentHashMap, ConcurrentLinkedQueue, 都是由Unsafe類來實現的。相對於與Java中的鎖,它基本無開銷,會原地等待。本文主要介紹下Unsafe中的主要操做。segmentfault

1 compareAndSwap

/**
* 比較obj的offset處內存位置中的值和指望的值,若是相同則更新。此更新是不可中斷的。
* 
* @param obj 須要更新的對象
* @param offset obj中整型field的偏移量
* @param expect 但願field中存在的值
* @param update 若是指望值expect與field的當前值相同,設置filed的值爲這個新值
* @return 若是field的值被更改返回true
*/
public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);

這個就是著名的CAS操做了,分爲三步來作數組

  1. 獲取obj對象中爲offset的偏移值,這裏假設爲realVal
  2. 比較realVal和expect
  3. 若是相同,將該值更新爲update,不然不更新

CAS家族還包括有,compareAndSwapObject(), compareAndSwapLong(), compareAndSwapInt()等等併發

用AtomicInteger中一個經典的例子來講明:this

public final int getAndAdd(int delta) {    
    return unsafe.getAndAddInt(this, valueOffset, delta);
}

//unsafe.getAndAddInt
public final int getAndAddInt(Object var1, long var2, int var4) {
    int var5;
    do {
    /**獲取原始值*/
        var5 = this.getIntVolatile(var1, var2);
    /**確認原始值沒有被其它線程修改時,再執行更新var5+var4操做,若是
        被其它線程修改過了,則會原地等待,持續循環*/
    } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));
    return var5;
}

這裏能夠看出CAS操做存在的問題,在低競態狀況下是ok的,可是在高競態狀況下,while循環會一直消耗cpu資源線程

2 putOrder

/***
   * Sets the value of the integer field at the specified offset in the
   * supplied object to the given value.  This is an ordered or lazy
   * version of <code>putIntVolatile(Object,long,int)</code>, which
   * doesn't guarantee the immediate visibility of the change to other
   * threads.  It is only really useful where the integer field is
   * <code>volatile</code>, and is thus expected to change unexpectedly.
   *
   * @param obj the object containing the field to modify.
   * @param offset the offset of the integer field within <code>obj</code>.
   * @param value the new value of the field.
   * @see #putIntVolatile(Object,long,int)
   */
  public native void putOrderedInt(Object obj, long offset, int value);

將obj對象的偏移量爲offset的位置修改成value,由於Java中沒有內存操做,而Unsafe的這個操做正好補充了內存操做的不足。也能夠用於數組操做,好比ConcurrentHashMap中就大量用到了該操做rest

Segment<K,V> s0 =
        new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
                         (HashEntry<K,V>[])new HashEntry[cap]);
    Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
    // 往數組下標爲0的位置,寫入s0: ss[0]=s0
    UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]

須要注意的是obj須要設置爲Volatile,不然對於其它線程會不可見code

該操做會插入一個#storestore的內存屏障,而非putXxxVolatile的#storeload的內存屏障,因此效率會高。所謂#storestore的內存屏障,是之虛擬機在對於這樣的語句Store1; StoreStore; Store2,在Store2及後續寫入操做執行前,保證Store1的寫入操做對其它處理器可見。對象

3 putXxxVolatile

/***
   * Sets the value of the integer field at the specified offset in the
   * supplied object to the given value, with volatile store semantics.
   *
   * @param obj the object containing the field to modify.
   * @param offset the offset of the integer field within <code>obj</code>.
   * @param value the new value of the field.
   */
  public native void putIntVolatile(Object obj, long offset, int value);

Volatile是會插入#storeload的內存屏障,效率略低內存

本篇爲俺的課堂《Java基礎:手寫jdk》的前置知識,歡迎你們圍觀ci

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