openjdk8 AtomicLong addAndGet()方法源碼分析

AtomicLong 類

​
 /**
     * Atomically adds the given value to the current value.
     *
     * @param delta the value to add
     * @return the updated value
     */
    public final long addAndGet(long delta) {
        return unsafe.getAndAddLong(this, valueOffset, delta) + delta;
    }

​

其中

AtomicLong.valueOffset變量

private static final long valueOffset;
static {
        try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicLong.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }

    private volatile long value;

    /**
     * Creates a new AtomicLong with the given initial value.
     *
     * @param initialValue the initial value
     */
    public AtomicLong(long initialValue) {
        value = initialValue;
    }

AtomicLong.unsafe變量

// setup to use Unsafe.compareAndSwapLong for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();

 

Unsafe類

/**
     * Atomically adds the given value to the current value of a field
     * or array element within the given object <code>o</code>
     * at the given <code>offset</code>.
     *
     * @param o object/array to update the field/element in
     * @param offset field/element offset
     * @param delta the value to add
     * @return the previous value
     * @since 1.8
     */
    public final long getAndAddLong(Object o, long offset, long delta) {
        long v;
        do {
            v = getLongVolatile(o, offset);
        } while (!compareAndSwapLong(o, offset, v, v + delta));
        return v;
    }

其中

Unsafe.getLongVolatile()方法

/** Volatile version of {@link #getLong(Object, long)}  */
    public native long    getLongVolatile(Object o, long offset);

Unsafe.compareAndSwapLong()方法

/**
     * Atomically update Java variable to <tt>x</tt> if it is currently
     * holding <tt>expected</tt>.
     * @return <tt>true</tt> if successful
     */
    public final native boolean compareAndSwapLong(Object o, long offset,
                                                   long expected,
                                                   long x);

查看java版本

[yeqiang@localhost ~]$ java -version
openjdk version "1.8.0_232"
OpenJDK Runtime Environment (build 1.8.0_232-b09)
OpenJDK 64-Bit Server VM (build 25.232-b09, mixed mode)

下載源代碼

https://github.com/openjdk/jdk/archive/jdk8-b09.tar.gz

./hotspot/src/share/vm/prims/unsafe.cpp

UNSAFE_ENTRY(jlong, Unsafe_GetLongVolatile(JNIEnv *env, jobject unsafe, jobject obj, jlong offset))
  UnsafeWrapper("Unsafe_GetLongVolatile");
  {
    if (VM_Version::supports_cx8()) {
      GET_FIELD_VOLATILE(obj, offset, jlong, v);
      return v;
    }
    else {
      Handle p (THREAD, JNIHandles::resolve(obj));
      jlong* addr = (jlong*)(index_oop_from_field_offset_long(p(), offset));
      ObjectLocker ol(p, THREAD);
      jlong value = *addr;
      return value;
    }
  }
UNSAFE_END

UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapLong(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jlong e, jlong x))
  UnsafeWrapper("Unsafe_CompareAndSwapLong");
  Handle p (THREAD, JNIHandles::resolve(obj));
  jlong* addr = (jlong*)(index_oop_from_field_offset_long(p(), offset));
  if (VM_Version::supports_cx8())
    return (jlong)(Atomic::cmpxchg(x, addr, e)) == e;
  else {
    jboolean success = false;
    ObjectLocker ol(p, THREAD);
    if (*addr == e) { *addr = x; success = true; }
    return success;
  }
UNSAFE_END

 

附：

關於Unsafe 資料https://segmentfault.com/a/1190000018037554?utm_source=tag-newest