AQS 框架之 Unsafe
■ 前言
之前 LockSupport 那篇已经叙述了是线程阻塞工具类，其底层由 Unsafe 实现，即 park(), unpark() 方法，获取指针偏移量，并操纵内存。本篇主要介绍 Unsafe 的源码，看看底层到底做了什么。
■ Unsafe 综述
- 作用： Unsafe是个后门类，封装了一些类似指针的操作，提供了一些可以直接操控内存和线程的底层操作
- 使用： Unsafe被JDK广泛用于nio包和并发包中，但是不建议在生产环境使用，风险太大
- 不安全： 不安全指的是指针的操作不安全(Java因此才把指针去掉)，若指针指错位置或计算指针偏移量出错，结果不可想象，比如说覆盖了别人的内存，那可能就GG思密达了…
- 补充1： 有机会开JVM番的话，笔者会从JVM源码角度重新解析Unsafe一些重要方法的实现
- 补充2： 此番为 AQS 框架之综述 (赶制中) 的子番
■ Unsafe 数据结构
1. 类定义
public final class Unsafe
2. 构造器
//私有构造器 --单例模式 private Unsafe() {}
3. 重要变量
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(); static { registerNatives(); Reflection.registerMethodsToFilter(Unsafe.class, new String[]{"getUnsafe"}); theUnsafe = new Unsafe();//单例模式 -饿汉式 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(); }
4. 重要方法
//获得给定对象内存偏移量的int值 public native int getInt(Object var1, long var2); //设置给定对象内存偏移量的int值 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); //....还有Boolean、Byte、Char、Short、Long、Float、Double的get\set.... //内存分配、释放 //分配内存 public native long allocateMemory(long var1); //扩充内存 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); } //拷贝内存 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); } //释放内存 public native void freeMemory(long var1); //获取字段在对象中的内存偏移量 public native long staticFieldOffset(Field var1); public native long objectFieldOffset(Field var1); public native Object staticFieldBase(Field var1); public native void ensureClassInitialized(Class<?> var1); //数组元素定位 //arrayBaseOffset 和 arrayIndexScale 搭配使用可以定位数组中每个元素在内存中的位置 //获取数组第一个元素的偏移地址 public native int arrayBaseOffset(Class<?> var1); //获取数组的转换因子，也就是数组中元素的增量地址 public native int arrayIndexScale(Class<?> var1); public native int addressSize(); public native int pageSize(); //类定义 public native Class<?> defineClass(String var1, byte[] var2, int var3, int var4, ClassLoader var5, ProtectionDomain var6); public native Class<?> defineClass(String var1, byte[] var2, int var3, int var4); public native Class<?> defineAnonymousClass(Class<?> var1, byte[] var2, Object[] var3); //创建实例 public native Object allocateInstance(Class<?> var1) throws InstantiationException; //Synchronized同步块的指令实现 1.8版的全部是@Deprecated public native void monitorEnter(Object var1); public native void monitorExit(Object var1); public native boolean tryMonitorEnter(Object var1); //异常抛出 public native void throwException(Throwable var1); //CAS操作 /** * 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 * @param obj 需要更新的对象 * @param offset obj中整型field的偏移量 * @param expect 希望field中存在的值 * @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 * @return 如果field的值被更改返回true */ public final native boolean compareAndSwapObject(Object obj, long offset, Object expect, Object update); 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); //获取给定对象的指定类型值，支持volatile load语义 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); //设置给定对象的int值，支持volatile load语义 public native void putIntVolatile(Object var1, long var2, int var4); //....还有Boolean、Byte、Char、Short、Long、Float、Double的volatile级别的get\put.... 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); //LockSupport类的原语支持-挂起和唤醒某个线程 public native void unpark(Object var1); public native void park(boolean var1, long var2); public native int getLoadAverage(double[] var1, int var2); //提供线程安全的add和set操作 public final int getAndAddInt(Object var1, long var2, int var4) { int var5; do { var5 = this.getIntVolatile(var1, var2); } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4)); return var5; } public final int getAndSetInt(Object var1, long var2, int var4) { int var5; do { var5 = this.getIntVolatile(var1, var2); } while(!this.compareAndSwapInt(var1, var2, var5, var4)); return var5; } //...还有Long和Object的线程安全的add和set操作... //栅栏支持 public native void loadFence(); public native void storeFence(); public native void fullFence();
5. 禁用的工厂方法
@CallerSensitive public static Unsafe getUnsafe() { Class var0 = Reflection.getCallerClass(); //该方法用于判断调用者的类加载器是否是系统核心加载器（即Bootstrap加载器） if(!VM.isSystemDomainLoader(var0.getClassLoader())) { throw new SecurityException("Unsafe"); } else { return theUnsafe; } }
- 我们先显性调用该工厂方法查看一下调用结果
package concurrent; import sun.misc.Unsafe; public class UnsafeDemo { public static void main(String[] args) { Unsafe.getUnsafe(); } } ------------------- //输出： Exception in thread "main" java.lang.SecurityException: Unsafe at sun.misc.Unsafe.getUnsafe(Unsafe.java:90) at concurrent.UnsafeDemo.main(UnsafeDemo.java:7) //分析：可以发现直接调用的话会直接抛出安全异常，原因是类加载器是AppClassLoader而并非是BootstrapLoader
- 根据Java 类加载器的工作原理，应用程序的类由AppLoader加载，而系统核心类由BootstrapLoader加载
- 当一个类的类加载器为null时，说明它是由BootstrapLoader加载的，即此类是系统核心类(比如rt.jar包中的类)
- 当一个类无法被BootstrapLoader加载时，其类加载器通常为AppClassLoader，即是属于自定义类
■ Unsafe 反射获取
/** * 我们可以通过反射机制获取Unsafe 的一个实例 */ public static Unsafe getUnsafe(){ try { //通过反射获取Unsafe的theUnsafe变量，即Unsafe实例对象 Field f = Unsafe.class.getDeclaredField("theUnsafe"); f.setAccessible(true); //注意field是static属性 //参见：private static final Unsafe theUnsafe; return (Unsafe) f.get(null); } catch (Exception e) { e.printStackTrace(); } return null; }
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