一篇看懂Java中的Unsafe类

import java.lang.reflect.Field; import sun.misc.Unsafe; public class UnsafePlayer {  public static void main(String[] args) throws Exception {  //通过反射实例化Unsafe  Field f = Unsafe.class.getDeclaredField("theUnsafe");  f.setAccessible(true);  Unsafe unsafe = (Unsafe) f.get(null);  //实例化Player  Player player = (Player) unsafe.allocateInstance(Player.class);  player.setName("li lei");  System.out.println(player.getName());  } }  class Player{  private String name;  private Player(){} public String getName() {  return name;  }  public void setName(String name) {  this.name = name;  } }

/ Unsafe mechanicsprivate static final sun.misc.Unsafe UNSAFE;private static final long headOffset;static { try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class<?> k = TransferStack.class; headOffset = UNSAFE.objectFieldOffset  (k.getDeclaredField("head")); } catch (Exception e) { throw new Error(e); }}//栈顶volatile SNode head;//更新栈顶boolean casHead(SNode h, SNode nh) { return h == head && UNSAFE.compareAndSwapObject(this, headOffset, h, nh);}

//静态属性的偏移量，用于在对应的Class对象中读写静态属性public native long staticFieldOffset(Field f); public native Object staticFieldBase(Field f);//判断是否需要初始化一个类public native boolean shouldBeInitialized(Class<?> c);//确保类被初始化public native void ensureClassInitialized(Class<?> c);//定义一个类，可用于动态创建类public native Class<?> defineClass(String name, byte[] b, int off, int len,     ClassLoader loader,     ProtectionDomain protectionDomain);//定义一个匿名类，可用于动态创建类public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches);

//获得对象的字段偏移量 public native long objectFieldOffset(Field f); //获得给定对象地址偏移量的int值public native int getInt(Object o, long offset);//设置给定对象地址偏移量的int值public native void putInt(Object o, long offset, int x);

//创建对象，但并不会调用其构造方法。如果类未被初始化，将初始化类。public native Object allocateInstance(Class<?> cls) throws InstantiationException;

/** * Report the offset of the first element in the storage allocation of a * given array class. If {@link #arrayIndexScale} returns a non-zero value * for the same class, you may use that scale factor, together with this * base offset, to form new offsets to access elements of arrays of the * given class. * * @see #getInt(Object, long) * @see #putInt(Object, long, int) *///返回数组中第一个元素的偏移地址public native int arrayBaseOffset(Class<?> arrayClass);//boolean、byte、short、char、int、long、float、double，及对象类型均有以下方法/** The value of {@code arrayBaseOffset(boolean[].class)} */public static final int ARRAY_BOOLEAN_BASE_OFFSET = theUnsafe.arrayBaseOffset(boolean[].class); /** * Report the scale factor for addressing elements in the storage * allocation of a given array class. However, arrays of "narrow" types * will generally not work properly with accessors like {@link * #getByte(Object, int)}, so the scale factor for such classes is reported * as zero. * * @see #arrayBaseOffset * @see #getInt(Object, long) * @see #putInt(Object, long, int) *///返回数组中每一个元素占用的大小public native int arrayIndexScale(Class<?> arrayClass); //boolean、byte、short、char、int、long、float、double，及对象类型均有以下方法/** The value of {@code arrayIndexScale(boolean[].class)} */public static final int ARRAY_BOOLEAN_INDEX_SCALE = theUnsafe.arrayIndexScale(boolean[].class);

//更新变量值为x，如果当前值为expected//o：对象 offset：偏移量 expected：期望值 x：新值public final native boolean compareAndSwapObject(Object o, long offset,       Object expected,       Object x); public final native boolean compareAndSwapInt(Object o, long offset,      int expected,      int x); public final native boolean compareAndSwapLong(Object o, long offset,      long expected,      long x);

//增加public final int getAndAddInt(Object o, long offset, int delta) { int v; do { v = getIntVolatile(o, offset); } while (!compareAndSwapInt(o, offset, v, v + delta)); return v;} 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;}//设置public final int getAndSetInt(Object o, long offset, int newValue) { int v; do { v = getIntVolatile(o, offset); } while (!compareAndSwapInt(o, offset, v, newValue)); return v;} public final long getAndSetLong(Object o, long offset, long newValue) { long v; do { v = getLongVolatile(o, offset); } while (!compareAndSwapLong(o, offset, v, newValue)); return v;} public final Object getAndSetObject(Object o, long offset, Object newValue) { Object v; do { v = getObjectVolatile(o, offset); } while (!compareAndSwapObject(o, offset, v, newValue)); return v;

//取消阻塞线程public native void unpark(Object thread);//阻塞线程public native void park(boolean isAbsolute, long time);//获得对象锁public native void monitorEnter(Object o);//释放对象锁public native void monitorExit(Object o);//尝试获取对象锁，返回true或false表示是否获取成功public native boolean tryMonitorEnter(Object o);

//从对象的指定偏移量处获取变量的引用，使用volatile的加载语义//相当于getObject(Object, long)的volatile版本public native Object getObjectVolatile(Object o, long offset); //存储变量的引用到对象的指定的偏移量处，使用volatile的存储语义//相当于putObject(Object, long, Object)的volatile版本public native void putObjectVolatile(Object o, long offset, Object x);

/** * Version of {@link #putObjectVolatile(Object, long, Object)} * that does not guarantee immediate visibility of the store to * other threads. This method is generally only useful if the * underlying field is a Java volatile (or if an array cell, one * that is otherwise only accessed using volatile accesses). */public native void putOrderedObject(Object o, long offset, Object x); /** Ordered/Lazy version of {@link #putIntVolatile(Object, long, int)} */public native void putOrderedInt(Object o, long offset, int x); /** Ordered/Lazy version of {@link #putLongVolatile(Object, long, long)} */public native void putOrderedLong(Object o, long offset, long x);

//内存屏障，禁止load操作重排序，即屏障前的load操作不能被重排序到屏障后，屏障后的load操作不能被重排序到屏障前public native void loadFence();//内存屏障，禁止store操作重排序，即屏障前的store操作不能被重排序到屏障后，屏障后的store操作不能被重排序到屏障前public native void storeFence();//内存屏障，禁止load、store操作重排序public native void fullFence();

//（boolean、byte、char、short、int、long、float、double)都有以下get、put两个方法。 //获得给定地址上的int值public native int getInt(long address);//设置给定地址上的int值public native void putInt(long address, int x);//获得本地指针public native long getAddress(long address);//存储本地指针到给定的内存地址public native void putAddress(long address, long x); //分配内存public native long allocateMemory(long bytes);//重新分配内存public native long reallocateMemory(long address, long bytes);//初始化内存内容public native void setMemory(Object o, long offset, long bytes, byte value);//初始化内存内容public void setMemory(long address, long bytes, byte value) { setMemory(null, address, bytes, value);}//内存内容拷贝public native void copyMemory(Object srcBase, long srcOffset,    Object destBase, long destOffset,    long bytes);//内存内容拷贝public void copyMemory(long srcAddress, long destAddress, long bytes) { copyMemory(null, srcAddress, null, destAddress, bytes);}//释放内存public native void freeMemory(long address);

//返回指针的大小。返回值为4或8。public native int addressSize(); /** The value of {@code addressSize()} */public static final int ADDRESS_SIZE = theUnsafe.addressSize(); //内存页的大小。public native int pageSize();