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 mechanics
private 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();