下面是Unsafe的主要功能：
- 普通读写：读写一个Object的field；直接从内存地址读写
- volatile读写：volatile读写Object的field，可以保证可见性和有序性
- 有序写：有序写Object的field，保证有序性不保证可见性
- 直接内存操作：申请内存；重新申请内存；释放内存；内存复制
- CAS相关：提供int，long，和Object的CAS操作
- 偏移量相关：获取对象属性和静态属性的偏移量；获取数组的arrayBaseOffset和arrayIndexScale
- 线程调度：挂起线程，唤醒线程，monitorEnter，monitorExit
- 类加载：定义类，创建对象，定义匿名内部类，确保一个类被加载，判断是否加载一个类
- 内存屏障：读屏障，写屏障，读写屏障
获取Unsafe实例
通过反射获取Unsafe实例：
public static Unsafe getUnsafe() throws IllegalAccessException {
Field unsafeField = Unsafe.class.getDeclaredFields()[0];
unsafeField.setAccessible(true);
Unsafe unsafe = (Unsafe) unsafeField.get(null);
return unsafe;
} 普通读写
// 读写一个Object属性的相关方法 public native int getInt(Object o, long l); public native void putInt(Object o, long l, int i); // 读写内存地址属性的相关方法 public native int getInt(long l); public native void putInt(long l, int i);
下面是测试用例：
package sun.misc;
import java.lang.reflect.Field;
/**
*
* @author zhaoxuyang
*/
public class UnsafeTest {
public static void main(String[] args) throws IllegalAccessException, NoSuchFieldException {
Model m = new Model(1, "zhangsan", 22);
sun.misc.Unsafe unsafe = getUnsafe();
System.out.println(unsafe.toString());
Class<?> c = Model.class;
long idOffset = unsafe.objectFieldOffset(c.getDeclaredField("id"));
long nameOffset = unsafe.objectFieldOffset(c.getDeclaredField("name"));
long ageOffset = unsafe.objectFieldOffset(c.getDeclaredField("age"));
System.out.println("idOffset: " + idOffset);
System.out.println("nameOffset: " + nameOffset);
System.out.println("ageOffset: " + ageOffset);
// 读写一个Object属性的相关方法
int getIntId = unsafe.getInt(m, idOffset);
int getIntName = unsafe.getInt(m, nameOffset);
int getIntAge = unsafe.getInt(m, ageOffset);
System.out.println(getIntId);
System.out.println(getIntName);
System.out.println(getIntAge);
unsafe.putInt(m, ageOffset, getIntAge + 1);
System.out.println(m.age);
}
public static Unsafe getUnsafe() throws IllegalAccessException {
Field unsafeField = Unsafe.class.getDeclaredFields()[0];
unsafeField.setAccessible(true);
Unsafe unsafe = (Unsafe) unsafeField.get(null);
return unsafe;
}
private static class Model {
long id;
String name;
int age;
Model(long id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}
}
} volatile读写
// 普通的读写无法保证可见性和有序性，而volatile读写就可以保证可见性和有序性。 public native int getIntVolatile(Object o, long l); public native void putIntVolatile(Object o, long l, int i);
有序写
// 有序写入只保证写入的有序性，不保证可见性，即一个线程的写入不保证其他线程立马可见，但是效率比volatile高。 public native void putOrderedInt(Object o, long l, int i);
直接内存操作
public native long allocateMemory(long l); // 分配内存 public native long reallocateMemory(long l, long l1); // 重新分配内存 public native void setMemory(Object o, long l, long l1, byte b); // 内存初始化 public native void copyMemory(Object o, long l, Object o1, long l1, long l2); // 内存复制 public native void freeMemory(long l); // 清除内存
compareAndSwap相关
Unsafe中提供了int,long和Object的CAS操作：
public final native boolean compareAndSwapObject(Object o, long l, Object o1, Object o2); public final native boolean compareAndSwapInt(Object o, long l, int i, int i1); public final native boolean compareAndSwapLong(Object o, long l, long l1, long l2);
CAS示例如下：
package sun.misc;
import java.lang.reflect.Field;
/**
*
* @author zhaoxuyang
*/
public class CasDemo{
private volatile int value;
private static final long valueOffset;
private final static Unsafe unsafe;
static {
try {
unsafe = getUnsafe();
valueOffset = unsafe.objectFieldOffset(CasDemo.class.getDeclaredField("value"));
} catch (Exception ex) {
throw new Error(ex);
}
}
private static Unsafe getUnsafe() throws IllegalAccessException {
Field unsafeField = Unsafe.class.getDeclaredFields()[0];
unsafeField.setAccessible(true);
Unsafe unsafe = (Unsafe) unsafeField.get(null);
return unsafe;
}
public final boolean cas(int expect, int update){
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}
public final int getAndIncr(){
return unsafe.getAndAddInt(this, valueOffset, 1);
}
public final int getAndDecr(){
return unsafe.getAndAddInt(this, valueOffset, -1);
}
public final int getValue(){
return value;
}
} 下面的例子将实现两个线程交替打印奇偶数：
package sun.misc;
/**
*
* @author zhaoxuyang
*/
public class CasDemoTest {
public static void main(String[] args) throws InterruptedException {
CasDemo casDemo = new CasDemo();
System.out.println(casDemo.getValue());
Thread t1 = new Thread(() -> {
while(true){
int value = casDemo.getValue();
if(value > 10){
break;
}
if(value % 2 == 1){
System.out.println("线程1 " + value);
casDemo.cas(value, value + 1);
}
}
});
Thread t2 = new Thread(() -> {
while(true){
int value = casDemo.getValue();
if(value > 10){
break;
}
if(value % 2 == 0){
System.out.println("线程2 " + value);
casDemo.cas(value, value + 1);
}
}
});
t1.start();
t2.start();
Thread.sleep(200);
System.out.println(casDemo.getValue());
}
} 输出如下：
run: 0 线程2 0 线程1 1 线程2 2 线程1 3 线程2 4 线程1 5 线程2 6 线程1 7 线程2 8 线程1 9 线程2 10 11
偏移量相关
public native long staticFieldOffset(Field field); // 获取静态属性Field在对象中的偏移量 public native long objectFieldOffset(Field field); // 非静态属性Field在对象实例中的偏移量 public native Object staticFieldBase(Field field); // 返回Field所在的对象 public native int arrayBaseOffset(Class<?> type); // 返回数组中第一个元素实际地址相对数组地址的偏移量 public native int arrayIndexScale(Class<?> type); //计算数组中第一个元素所占用的内存空间
线程调度
public native void park(boolean bln, long l); // 挂起线程 public native void unpark(Object o); // 唤醒线程 // 用于加锁，synchronized便是通过以下指令实现 @Deprecated public native void monitorEnter(Object o); @Deprecated public native void monitorExit(Object o); @Deprecated public native boolean tryMonitorEnter(Object o);
类加载
// 定义一个类，用于动态地创建类 public native Class<?> defineClass(String string, byte[] bytes, int i, int i1, ClassLoader cl, ProtectionDomain pd); // 用于动态地创建一个匿名内部类 public native Class<?> defineAnonymousClass(Class<?> type, byte[] bytes, Object[] os); // 用于创建一个类的实例，但是不会调用这个实例的构造方法，如果这个类还未被初始化，则初始化这个类。 public native Object allocateInstance(Class<?> type) throws InstantiationException; // 用于判断是否需要初始化一个类 public native boolean shouldBeInitialized(Class<?> type); // 用于保证已经初始化过一个类 public native void ensureClassInitialized(Class<?> type);
内存屏障
public native void loadFence(); // 保证在这个屏障之前的所有【读】操作都已经完成 public native void storeFence(); // 【写】 public native void fullFence(); // 【读写】
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