Unsafe在JUC里用的非常普遍,所以花时间看了看其实现, 贴先源码吧,其实并不复杂:
sun.misc.Unsafe.java 源码:
http://www.javasourcecode.org/html/open-source/jdk/jdk-6u23/sun/misc/Unsafe.java.html
从源码实现来看,Unsafe是个单例,其实例只能通过静态方法获得:
public static Unsafe getUnsafe() { Class cc = sun.reflect.Reflection.getCallerClass(2); if (cc.getClassLoader() != null) throw new SecurityException("Unsafe"); return theUnsafe; }
如果我们正常的应用程序去调用的话,会抛出SecurityException,获取实例时先看调用的类是否启动类加载器加载,不是的话就抛出异常,其comment也强调Unsafe是应用于安全性代码的.那如果我们的确需要使用时,可以通过反射,将私有变量theUnsafe(唯一实例的引用)可见,再获取:
Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe"); theUnsafe.setAccessible(true); Unsafe unsafe = (Unsafe) theUnsafe.get(null);
Unsafe 也是基于CAS(compare and swap)的,CAS是现代CPU提供给并发程序使用的原语操作. 不同的CPU有不同的使用规范.在 Intel 处理器中,比较并交换通过指令的 cmpxchg 系列实现。CAS 操作包含三个操作数 —— 内存位置(V)、预期原值(A)和新值(B).JUC里的原子类的实现都是通过CAS,CAS能更高效的从机器码层面提供并发操作.
另,相应的sun.misc.natUnsafe.cc 源码:
#include <gcj/cni.h> #include <gcj/field.h> #include <gcj/javaprims.h> #include <jvm.h> #include <sun/misc/Unsafe.h> #include <java/lang/System.h> #include <java/lang/InterruptedException.h> #include <java/lang/Thread.h> #include <java/lang/Long.h> #include "sysdep/locks.h" // Use a spinlock for multi-word accesses class spinlock { static volatile obj_addr_t lock; public: spinlock () { while (! compare_and_swap (&lock, 0, 1)) _Jv_ThreadYield (); } ~spinlock () { release_set (&lock, 0); } }; // This is a single lock that is used for all synchronized accesses if // the compiler can't generate inline compare-and-swap operations. In // most cases it'll never be used, but the i386 needs it for 64-bit // locked accesses and so does PPC32. It's worth building libgcj with // target=i486 (or above) to get the inlines. volatile obj_addr_t spinlock::lock; static inline bool compareAndSwap (volatile jint *addr, jint old, jint new_val) { jboolean result = false; spinlock lock; if ((result = (*addr == old))) *addr = new_val; return result; } static inline bool compareAndSwap (volatile jlong *addr, jlong old, jlong new_val) { jboolean result = false; spinlock lock; if ((result = (*addr == old))) *addr = new_val; return result; } static inline bool compareAndSwap (volatile jobject *addr, jobject old, jobject new_val) { jboolean result = false; spinlock lock; if ((result = (*addr == old))) *addr = new_val; return result; } jlong sun::misc::Unsafe::objectFieldOffset (::java::lang::reflect::Field *field) { _Jv_Field *fld = _Jv_FromReflectedField (field); // FIXME: what if it is not an instance field? return fld->getOffset(); } jint sun::misc::Unsafe::arrayBaseOffset (jclass arrayClass) { // FIXME: assert that arrayClass is array. jclass eltClass = arrayClass->getComponentType(); return (jint)(jlong) _Jv_GetArrayElementFromElementType (NULL, eltClass); } jint sun::misc::Unsafe::arrayIndexScale (jclass arrayClass) { // FIXME: assert that arrayClass is array. jclass eltClass = arrayClass->getComponentType(); if (eltClass->isPrimitive()) return eltClass->size(); return sizeof (void *); } // These methods are used when the compiler fails to generate inline // versions of the compare-and-swap primitives. jboolean sun::misc::Unsafe::compareAndSwapInt (jobject obj, jlong offset, jint expect, jint update) { jint *addr = (jint *)((char *)obj + offset); return compareAndSwap (addr, expect, update); } jboolean sun::misc::Unsafe::compareAndSwapLong (jobject obj, jlong offset, jlong expect, jlong update) { volatile jlong *addr = (jlong*)((char *) obj + offset); return compareAndSwap (addr, expect, update); } jboolean sun::misc::Unsafe::compareAndSwapObject (jobject obj, jlong offset, jobject expect, jobject update) { jobject *addr = (jobject*)((char *) obj + offset); return compareAndSwap (addr, expect, update); } void sun::misc::Unsafe::putOrderedInt (jobject obj, jlong offset, jint value) { volatile jint *addr = (jint *) ((char *) obj + offset); *addr = value; } void sun::misc::Unsafe::putOrderedLong (jobject obj, jlong offset, jlong value) { volatile jlong *addr = (jlong *) ((char *) obj + offset); spinlock lock; *addr = value; } void sun::misc::Unsafe::putOrderedObject (jobject obj, jlong offset, jobject value) { volatile jobject *addr = (jobject *) ((char *) obj + offset); *addr = value; } void sun::misc::Unsafe::putIntVolatile (jobject obj, jlong offset, jint value) { write_barrier (); volatile jint *addr = (jint *) ((char *) obj + offset); *addr = value; } void sun::misc::Unsafe::putLongVolatile (jobject obj, jlong offset, jlong value) { volatile jlong *addr = (jlong *) ((char *) obj + offset); spinlock lock; *addr = value; } void sun::misc::Unsafe::putObjectVolatile (jobject obj, jlong offset, jobject value) { write_barrier (); volatile jobject *addr = (jobject *) ((char *) obj + offset); *addr = value; } #if 0 // FIXME void sun::misc::Unsafe::putInt (jobject obj, jlong offset, jint value) { jint *addr = (jint *) ((char *) obj + offset); *addr = value; } #endif void sun::misc::Unsafe::putLong (jobject obj, jlong offset, jlong value) { jlong *addr = (jlong *) ((char *) obj + offset); spinlock lock; *addr = value; } void sun::misc::Unsafe::putObject (jobject obj, jlong offset, jobject value) { jobject *addr = (jobject *) ((char *) obj + offset); *addr = value; } jint sun::misc::Unsafe::getIntVolatile (jobject obj, jlong offset) { volatile jint *addr = (jint *) ((char *) obj + offset); jint result = *addr; read_barrier (); return result; } jobject sun::misc::Unsafe::getObjectVolatile (jobject obj, jlong offset) { volatile jobject *addr = (jobject *) ((char *) obj + offset); jobject result = *addr; read_barrier (); return result; } jlong sun::misc::Unsafe::getLong (jobject obj, jlong offset) { jlong *addr = (jlong *) ((char *) obj + offset); spinlock lock; return *addr; } jlong sun::misc::Unsafe::getLongVolatile (jobject obj, jlong offset) { volatile jlong *addr = (jlong *) ((char *) obj + offset); spinlock lock; return *addr; } void sun::misc::Unsafe::unpark (::java::lang::Thread *thread) { natThread *nt = (natThread *) thread->data; nt->park_helper.unpark (); } void sun::misc::Unsafe::park (jboolean isAbsolute, jlong time) { using namespace ::java::lang; Thread *thread = Thread::currentThread(); natThread *nt = (natThread *) thread->data; nt->park_helper.park (isAbsolute, time); }