openjdk8 AtomicLong addAndGet()方法源码分析

不念不忘少年蓝@ 2023-02-13 08:53 21阅读 0赞

AtomicLong 类

/**
         * 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变量

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变量

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

Unsafe类

/**
         * 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)

下载源代码

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

关于Unsafe 资料[https://segmentfault.com/a/1190000018037554?utm\_source=tag-newest][https_segmentfault.com_a_1190000018037554_utm_source_tag-newest]

[https_github.com_openjdk_jdk_archive_jdk8-b09.tar.gz]: https://github.com/openjdk/jdk/archive/jdk8-b09.tar.gz
[https_segmentfault.com_a_1190000018037554_utm_source_tag-newest]: https://segmentfault.com/a/1190000018037554?utm_source=tag-newest