sun.misc.Unsafe实例介绍

    java.util.concurrent 包下很多类的实现用到了基于硬件的CAS算法， 不再使用synchronized关键来进行并发控制,查看相关的源码后会发现最后都指向了一个类 ，这就是SUN未开源的sun.misc.Unsafe的类，该类功能很强大，涉及到类加载机制，其实例一般情况是获取不到的。还好有个反射的机制可以变通下，因为JVM 加载rt.jar时已经加载了该类，并且初始化过。上代码 

/* * Copyright 2010, the original author or authors.  All rights reserved. */package com.api;import java.lang.reflect.Field;import org.apache.log4j.Logger;import sun.misc.Unsafe;/** * 获取{@link sun.misc.Unsafe}实例<br> * 从rt.jar包中反编译该类获取的信息 *  * <pre> *  * package sun.misc; *  *  import java.lang.reflect.Field; *  import java.lang.reflect.Modifier; *  import java.security.ProtectionDomain; *  import sun.reflect.Reflection; *  *  public final class Unsafe *  { *  private static final Unsafe theUnsafe; *  public static final int INVALID_FIELD_OFFSET = -1; *  *  private static native void registerNatives(); *  *  public static Unsafe getUnsafe() *  { *  Class localClass = Reflection.getCallerClass(2); *  if (localClass.getClassLoader() != null) *  throw new SecurityException(&quot;Unsafe&quot;); *  return theUnsafe; *  } * static *  { *  registerNatives(); *  *  theUnsafe = new Unsafe(); *  } * .......................... * </pre> *  * @author <a href="mailto:[e-mail]">xiao jiang</a> * @version %I%, %G% * @history 2010-12-23 */public class UnsafeSupport {  private static Logger log = Logger.getLogger(UnsafeSupport.class);  private static Unsafe unsafe;  static {    Field field;    try {      // 由反编译Unsafe类获得的信息      field = Unsafe.class.getDeclaredField("theUnsafe");      field.setAccessible(true);      // 获取静态属性,Unsafe在启动JVM时随rt.jar装载      unsafe = (Unsafe) field.get(null);    } catch (Exception e) {      log.error("Get Unsafe instance occur error", e);    }  }  /**   * 获取{@link Unsafe }   */  public static Unsafe getInstance() {    return unsafe;  }  public static void main(String[] args) {  }}

搞个测试类玩玩

/* *  Copyright 2010, the original author or authors.  All rights reserved. */package com.api;/** * 分析目标 *  * @author <a href="mailto:[e-mail]">xiao jiang</a> * @version %I%, %G% * @history 2010-12-23 */public class AnalyzedTarget {  private byte byteParam;  private char charParam;  private short shorteParam;  private int intParam;  private int intParam2;  private long longParam;  private double doubleParam;  private String strParam;  private String strParam2;  }

运行测试案例

/* *Copyright 2010, the original author or authors.  All rights reserved. */package com.api;import java.lang.reflect.Field;import org.apache.log4j.Logger;import sun.misc.Unsafe;/** * 通过反射获取rt.jar下的Unsafe类 *  * @author <a href="mailto:[e-mail]">xiao jiang</a> * @version %I%, %G% * @history 2010-12-23 */public class UnsafeTest {  private static Logger log = Logger.getLogger(UnsafeTest.class);  public static void main(String[] args) {    Unsafe unsafe = UnsafeSupport.getInstance();    // 这个很牛    // unsafe.allocateMemory(1000000000l);    Class clazz = AnalyzedTarget.class;    Field[] fields = clazz.getDeclaredFields();    log.info("fieldName:fieldOffset");    // 获取属性偏移量，可以通过这个偏移量给属性设置    for (Field f : fields) {      log.info(f.getName() + ":" + unsafe.objectFieldOffset(f));    }    // arg0, arg1, arg2, arg3 分别是目标对象实例，目标对象属性偏移量，当前预期值，要设的值    // unsafe.compareAndSwapInt(arg0, arg1, arg2, arg3)    AnalyzedTarget target = new AnalyzedTarget();    // 偏移量编译后一般不会变的,intParam这个属性的偏移量    long intParamOffset = 24;    // 给它设置,返回true表明设置成功, 基于有名的CAS算法的方法，并发包用这个方法很多    log.info(unsafe.compareAndSwapInt(target, intParamOffset, 0, 3));    //比较失败    log.info(unsafe.compareAndSwapInt(target, intParamOffset, 0, 10));    //验证下上面是否设置成功,应该还是3，返回ture说明上面没该    log.info(unsafe.compareAndSwapInt(target, intParamOffset, 3, 10));  }}

结果
2010-12-23 16:59:56:352 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:29) - fieldName:fieldOffset
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:32) - byteParam:36
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:32) - charParam:32
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:32) - shorteParam:34
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:32) - intParam:24
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:32) - intParam2:28
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:32) - longParam:8
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:32) - doubleParam:16
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:32) - strParam:40
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:32) - strParam2:44
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:40) - true
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:42) - false
2010-12-23 16:59:56:368 [INFO]  com.api.UnsafeTest.main(UnsafeTest.java:44) - true

转载地址:http://xiao-jiang51.iteye.com/blog/850413