码迷，mamicode.com
首页 > 编程语言 > 详细

[java] java 中Unsafe类学习

时间：2016-01-06 23:34:35      阅读：286      评论：0      收藏：0      [点我收藏+]

标签：

java不能直接访问操作系统底层，而是通过本地方法来访问。Unsafe类提供了硬件级别的原子操作，主要提供了以下功能：

1、通过Unsafe类可以分配内存，可以释放内存；

类中提供的3个本地方法allocateMemory、reallocateMemory、freeMemory分别用于分配内存，扩充内存和释放内存，与C语言中的3个方法对应。

2、可以定位对象某字段的内存位置，也可以修改对象的字段值，即使它是私有的；
 
  1. public native long allocateMemory(long l);  
  2. public native long reallocateMemory(long l, long l1);  
  3. public native void freeMemory(long l);  
字段的定位：
JAVA中对象的字段的定位可能通过staticFieldOffset方法实现，该方法返回给定field的内存地址偏移量，这个值对于给定的filed是唯一的且是固定不变的。
getIntVolatile方法获取对象中offset偏移地址对应的整型field的值,支持volatile load语义。
getLong方法获取对象中offset偏移地址对应的long型field的值
数组元素定位：
Unsafe类中有很多以BASE_OFFSET结尾的常量，比如ARRAY_INT_BASE_OFFSET，ARRAY_BYTE_BASE_OFFSET等，这些常量值是通过arrayBaseOffset方法得到的。arrayBaseOffset方法是一个本地方法，可以获取数组第一个元素的偏移地址。Unsafe类中还有很多以INDEX_SCALE结尾的常量，比如 ARRAY_INT_INDEX_SCALE ， ARRAY_BYTE_INDEX_SCALE等，这些常量值是通过arrayIndexScale方法得到的。arrayIndexScale方法也是一个本地方法，可以获取数组的转换因子，也就是数组中元素的增量地址。将arrayBaseOffset与arrayIndexScale配合使用，可以定位数组中每个元素在内存中的位置。
[java] view plaincopyprint?
 
  1. public final class Unsafe {  
  2.     public static final int ARRAY_INT_BASE_OFFSET;  
  3.     public static final int ARRAY_INT_INDEX_SCALE;  
  4.   
  5.     public native long staticFieldOffset(Field field);  
  6.     public native int getIntVolatile(Object obj, long l);  
  7.     public native long getLong(Object obj, long l);  
  8.     public native int arrayBaseOffset(Class class1);  
  9.     public native int arrayIndexScale(Class class1);  
  10.   
  11.     static   
  12.     {  
  13.         ARRAY_INT_BASE_OFFSET = theUnsafe.arrayBaseOffset([I);  
  14.         ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale([I);  
  15.     }  
  16. }  

3、挂起与恢复

将一个线程进行挂起是通过park方法实现的，调用 park后，线程将一直阻塞直到超时或者中断等条件出现。unpark可以终止一个挂起的线程，使其恢复正常。整个并发框架中对线程的挂起操作被封装在 LockSupport类中，LockSupport类中有各种版本pack方法，但最终都调用了Unsafe.park()方法。
[java] view plaincopyprint?
 
  1. public class LockSupport {  
  2.     public static void unpark(Thread thread) {  
  3.         if (thread != null)  
  4.             unsafe.unpark(thread);  
  5.     }  
  6.   
  7.     public static void park(Object blocker) {  
  8.         Thread t = Thread.currentThread();  
  9.         setBlocker(t, blocker);  
  10.         unsafe.park(false, 0L);  
  11.         setBlocker(t, null);  
  12.     }  
  13.   
  14.     public static void parkNanos(Object blocker, long nanos) {  
  15.         if (nanos > 0) {  
  16.             Thread t = Thread.currentThread();  
  17.             setBlocker(t, blocker);  
  18.             unsafe.park(false, nanos);  
  19.             setBlocker(t, null);  
  20.         }  
  21.     }  
  22.   
  23.     public static void parkUntil(Object blocker, long deadline) {  
  24.         Thread t = Thread.currentThread();  
  25.         setBlocker(t, blocker);  
  26.         unsafe.park(true, deadline);  
  27.         setBlocker(t, null);  
  28.     }  
  29.   
  30.     public static void park() {  
  31.         unsafe.park(false, 0L);  
  32.     }  
  33.   
  34.     public static void parkNanos(long nanos) {  
  35.         if (nanos > 0)  
  36.             unsafe.park(false, nanos);  
  37.     }  
  38.   
  39.     public static void parkUntil(long deadline) {  
  40.         unsafe.park(true, deadline);  
  41.     }  
  42. }  
4、CAS操作
是通过compareAndSwapXXX方法实现的
[java] view plaincopyprint?
 
  1. /** 
  2. * 比较obj的offset处内存位置中的值和期望的值，如果相同则更新。此更新是不可中断的。 
  3. *  
  4. * @param obj 需要更新的对象 
  5. * @param offset obj中整型field的偏移量 
  6. * @param expect 希望field中存在的值 
  7. * @param update 如果期望值expect与field的当前值相同，设置filed的值为这个新值 
  8. * @return 如果field的值被更改返回true 
  9. */  
  10. public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);  
 
CAS操作有3个操作数，内存值M，预期值E，新值U，如果M==E，则将内存值修改为B，否则啥都不做。
 
另外附上Unsafe 源代码：
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package sun.misc;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.security.ProtectionDomain;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;

public final class Unsafe {
    private static final Unsafe theUnsafe;
    public static final int INVALID_FIELD_OFFSET = -1;
    public static final int ARRAY_BOOLEAN_BASE_OFFSET;
    public static final int ARRAY_BYTE_BASE_OFFSET;
    public static final int ARRAY_SHORT_BASE_OFFSET;
    public static final int ARRAY_CHAR_BASE_OFFSET;
    public static final int ARRAY_INT_BASE_OFFSET;
    public static final int ARRAY_LONG_BASE_OFFSET;
    public static final int ARRAY_FLOAT_BASE_OFFSET;
    public static final int ARRAY_DOUBLE_BASE_OFFSET;
    public static final int ARRAY_OBJECT_BASE_OFFSET;
    public static final int ARRAY_BOOLEAN_INDEX_SCALE;
    public static final int ARRAY_BYTE_INDEX_SCALE;
    public static final int ARRAY_SHORT_INDEX_SCALE;
    public static final int ARRAY_CHAR_INDEX_SCALE;
    public static final int ARRAY_INT_INDEX_SCALE;
    public static final int ARRAY_LONG_INDEX_SCALE;
    public static final int ARRAY_FLOAT_INDEX_SCALE;
    public static final int ARRAY_DOUBLE_INDEX_SCALE;
    public static final int ARRAY_OBJECT_INDEX_SCALE;
    public static final int ADDRESS_SIZE;

    private static native void registerNatives();

    private Unsafe() {
    }

    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class var0 = Reflection.getCallerClass();
        if(var0.getClassLoader() != null) {
            throw new SecurityException("Unsafe");
        } else {
            return theUnsafe;
        }
    }

    public native int getInt(Object var1, long var2);

    public native void putInt(Object var1, long var2, int var4);

    public native Object getObject(Object var1, long var2);

    public native void putObject(Object var1, long var2, Object var4);

    public native boolean getBoolean(Object var1, long var2);

    public native void putBoolean(Object var1, long var2, boolean var4);

    public native byte getByte(Object var1, long var2);

    public native void putByte(Object var1, long var2, byte var4);

    public native short getShort(Object var1, long var2);

    public native void putShort(Object var1, long var2, short var4);

    public native char getChar(Object var1, long var2);

    public native void putChar(Object var1, long var2, char var4);

    public native long getLong(Object var1, long var2);

    public native void putLong(Object var1, long var2, long var4);

    public native float getFloat(Object var1, long var2);

    public native void putFloat(Object var1, long var2, float var4);

    public native double getDouble(Object var1, long var2);

    public native void putDouble(Object var1, long var2, double var4);

    /** @deprecated */
    @Deprecated
    public int getInt(Object var1, int var2) {
        return this.getInt(var1, (long)var2);
    }

    /** @deprecated */
    @Deprecated
    public void putInt(Object var1, int var2, int var3) {
        this.putInt(var1, (long)var2, var3);
    }

    /** @deprecated */
    @Deprecated
    public Object getObject(Object var1, int var2) {
        return this.getObject(var1, (long)var2);
    }

    /** @deprecated */
    @Deprecated
    public void putObject(Object var1, int var2, Object var3) {
        this.putObject(var1, (long)var2, var3);
    }

    /** @deprecated */
    @Deprecated
    public boolean getBoolean(Object var1, int var2) {
        return this.getBoolean(var1, (long)var2);
    }

    /** @deprecated */
    @Deprecated
    public void putBoolean(Object var1, int var2, boolean var3) {
        this.putBoolean(var1, (long)var2, var3);
    }

    /** @deprecated */
    @Deprecated
    public byte getByte(Object var1, int var2) {
        return this.getByte(var1, (long)var2);
    }

    /** @deprecated */
    @Deprecated
    public void putByte(Object var1, int var2, byte var3) {
        this.putByte(var1, (long)var2, var3);
    }

    /** @deprecated */
    @Deprecated
    public short getShort(Object var1, int var2) {
        return this.getShort(var1, (long)var2);
    }

    /** @deprecated */
    @Deprecated
    public void putShort(Object var1, int var2, short var3) {
        this.putShort(var1, (long)var2, var3);
    }

    /** @deprecated */
    @Deprecated
    public char getChar(Object var1, int var2) {
        return this.getChar(var1, (long)var2);
    }

    /** @deprecated */
    @Deprecated
    public void putChar(Object var1, int var2, char var3) {
        this.putChar(var1, (long)var2, var3);
    }

    /** @deprecated */
    @Deprecated
    public long getLong(Object var1, int var2) {
        return this.getLong(var1, (long)var2);
    }

    /** @deprecated */
    @Deprecated
    public void putLong(Object var1, int var2, long var3) {
        this.putLong(var1, (long)var2, var3);
    }

    /** @deprecated */
    @Deprecated
    public float getFloat(Object var1, int var2) {
        return this.getFloat(var1, (long)var2);
    }

    /** @deprecated */
    @Deprecated
    public void putFloat(Object var1, int var2, float var3) {
        this.putFloat(var1, (long)var2, var3);
    }

    /** @deprecated */
    @Deprecated
    public double getDouble(Object var1, int var2) {
        return this.getDouble(var1, (long)var2);
    }

    /** @deprecated */
    @Deprecated
    public void putDouble(Object var1, int var2, double var3) {
        this.putDouble(var1, (long)var2, var3);
    }

    public native byte getByte(long var1);

    public native void putByte(long var1, byte var3);

    public native short getShort(long var1);

    public native void putShort(long var1, short var3);

    public native char getChar(long var1);

    public native void putChar(long var1, char var3);

    public native int getInt(long var1);

    public native void putInt(long var1, int var3);

    public native long getLong(long var1);

    public native void putLong(long var1, long var3);

    public native float getFloat(long var1);

    public native void putFloat(long var1, float var3);

    public native double getDouble(long var1);

    public native void putDouble(long var1, double var3);

    public native long getAddress(long var1);

    public native void putAddress(long var1, long var3);

    public native long allocateMemory(long var1);

    public native long reallocateMemory(long var1, long var3);

    public native void setMemory(Object var1, long var2, long var4, byte var6);

    public void setMemory(long var1, long var3, byte var5) {
        this.setMemory((Object)null, var1, var3, var5);
    }

    public native void copyMemory(Object var1, long var2, Object var4, long var5, long var7);

    public void copyMemory(long var1, long var3, long var5) {
        this.copyMemory((Object)null, var1, (Object)null, var3, var5);
    }

    public native void freeMemory(long var1);

    /** @deprecated */
    @Deprecated
    public int fieldOffset(Field var1) {
        return Modifier.isStatic(var1.getModifiers())?(int)this.staticFieldOffset(var1):(int)this.objectFieldOffset(var1);
    }

    /** @deprecated */
    @Deprecated
    public Object staticFieldBase(Class var1) {
        Field[] var2 = var1.getDeclaredFields();

        for(int var3 = 0; var3 < var2.length; ++var3) {
            if(Modifier.isStatic(var2[var3].getModifiers())) {
                return this.staticFieldBase(var2[var3]);
            }
        }

        return null;
    }

    public native long staticFieldOffset(Field var1);

    public native long objectFieldOffset(Field var1);

    public native Object staticFieldBase(Field var1);

    public native boolean shouldBeInitialized(Class<?> var1);

    public native void ensureClassInitialized(Class var1);

    public native int arrayBaseOffset(Class var1);

    public native int arrayIndexScale(Class var1);

    public native int addressSize();

    public native int pageSize();

    public native Class defineClass(String var1, byte[] var2, int var3, int var4, ClassLoader var5, ProtectionDomain var6);

    /** @deprecated */
    @Deprecated
    @CallerSensitive
    public native Class defineClass(String var1, byte[] var2, int var3, int var4);

    public native Class defineAnonymousClass(Class var1, byte[] var2, Object[] var3);

    public native Object allocateInstance(Class var1) throws InstantiationException;

    public native void monitorEnter(Object var1);

    public native void monitorExit(Object var1);

    public native boolean tryMonitorEnter(Object var1);

    public native void throwException(Throwable var1);

    public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);

    public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);

    public final native boolean compareAndSwapLong(Object var1, long var2, long var4, long var6);

    public native Object getObjectVolatile(Object var1, long var2);

    public native void putObjectVolatile(Object var1, long var2, Object var4);

    public native int getIntVolatile(Object var1, long var2);

    public native void putIntVolatile(Object var1, long var2, int var4);

    public native boolean getBooleanVolatile(Object var1, long var2);

    public native void putBooleanVolatile(Object var1, long var2, boolean var4);

    public native byte getByteVolatile(Object var1, long var2);

    public native void putByteVolatile(Object var1, long var2, byte var4);

    public native short getShortVolatile(Object var1, long var2);

    public native void putShortVolatile(Object var1, long var2, short var4);

    public native char getCharVolatile(Object var1, long var2);

    public native void putCharVolatile(Object var1, long var2, char var4);

    public native long getLongVolatile(Object var1, long var2);

    public native void putLongVolatile(Object var1, long var2, long var4);

    public native float getFloatVolatile(Object var1, long var2);

    public native void putFloatVolatile(Object var1, long var2, float var4);

    public native double getDoubleVolatile(Object var1, long var2);

    public native void putDoubleVolatile(Object var1, long var2, double var4);

    public native void putOrderedObject(Object var1, long var2, Object var4);

    public native void putOrderedInt(Object var1, long var2, int var4);

    public native void putOrderedLong(Object var1, long var2, long var4);

    public native void unpark(Object var1);

    public native void park(boolean var1, long var2);

    public native int getLoadAverage(double[] var1, int var2);

    static {
        registerNatives();
        Reflection.registerMethodsToFilter(Unsafe.class, new String[]{"getUnsafe"});
        theUnsafe = new Unsafe();
        ARRAY_BOOLEAN_BASE_OFFSET = theUnsafe.arrayBaseOffset(boolean[].class);
        ARRAY_BYTE_BASE_OFFSET = theUnsafe.arrayBaseOffset(byte[].class);
        ARRAY_SHORT_BASE_OFFSET = theUnsafe.arrayBaseOffset(short[].class);
        ARRAY_CHAR_BASE_OFFSET = theUnsafe.arrayBaseOffset(char[].class);
        ARRAY_INT_BASE_OFFSET = theUnsafe.arrayBaseOffset(int[].class);
        ARRAY_LONG_BASE_OFFSET = theUnsafe.arrayBaseOffset(long[].class);
        ARRAY_FLOAT_BASE_OFFSET = theUnsafe.arrayBaseOffset(float[].class);
        ARRAY_DOUBLE_BASE_OFFSET = theUnsafe.arrayBaseOffset(double[].class);
        ARRAY_OBJECT_BASE_OFFSET = theUnsafe.arrayBaseOffset(Object[].class);
        ARRAY_BOOLEAN_INDEX_SCALE = theUnsafe.arrayIndexScale(boolean[].class);
        ARRAY_BYTE_INDEX_SCALE = theUnsafe.arrayIndexScale(byte[].class);
        ARRAY_SHORT_INDEX_SCALE = theUnsafe.arrayIndexScale(short[].class);
        ARRAY_CHAR_INDEX_SCALE = theUnsafe.arrayIndexScale(char[].class);
        ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale(int[].class);
        ARRAY_LONG_INDEX_SCALE = theUnsafe.arrayIndexScale(long[].class);
        ARRAY_FLOAT_INDEX_SCALE = theUnsafe.arrayIndexScale(float[].class);
        ARRAY_DOUBLE_INDEX_SCALE = theUnsafe.arrayIndexScale(double[].class);
        ARRAY_OBJECT_INDEX_SCALE = theUnsafe.arrayIndexScale(Object[].class);
        ADDRESS_SIZE = theUnsafe.addressSize();
    }
}

 

[java] java 中Unsafe类学习

标签：

原文地址：http://www.cnblogs.com/lonelywolfmoutain/p/5107570.html
踩
(0)
赞
(0)
   
举报
评论 一句话评论（0）
登录后才能评论！
分享档案
更多>
2021年07月29日 (22)
2021年07月28日 (40)
2021年07月27日 (32)
2021年07月26日 (79)
2021年07月23日 (29)
2021年07月22日 (30)
2021年07月21日 (42)
2021年07月20日 (16)
2021年07月19日 (90)
2021年07月16日 (35)
周排行
mamicode.com排行更多图片
更多
友情链接
兰亭集智   国之画   百度统计   站长统计   阿里云   chrome插件   新版天听网
关于我们 - 联系我们 - 留言反馈
© 2014 mamicode.com 版权所有  联系我们:gaon5@hotmail.com
迷上了代码！