Java中的Unsafe类111

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);

2.4.4内存屏障相关

Java 8引入 ，用于定义内存屏障，避免代码重排序。

2.5直接内存访问（非堆内存）

allocateMemory所分配的内存需要手动free（不被GC回收）

2.6系统相关

3.参考资料

https://www.cnblogs.com/pkufork/p/java_unsafe.html 说一说Java中的Unsafe类

https://www.cnblogs.com/suxuan/p/4948608.html java魔法类：sun.misc.Unsafe