sun.misc.Unsafe的几个点

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一.获取Unsafe，通用的办法利用反射机制

    Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
    unsafeField.setAccessible(true);
    Unsafe unsafe = (Unsafe) unsafeField.get(null);

二.Unsafe自己对调用者的检查

    public static Unsafe getUnsafe() {
        Class cc = sun.reflect.Reflection.getCallerClass(2);
        if (cc.getClassLoader() != null)
            throw new SecurityException("Unsafe");
        return theUnsafe;
}

getCallerClass调用栈帧，从0开始。
0：getCallerClass方法的Reflection类；
1：getUnsafe方法的Unsafe类；
2：调用getUnsafe方法的所属类
比如：

   public static void main(String[] args) throws Exception {
         Class cc = sun.reflect.Reflection.getCallerClass(1); 
         //0：getCallerClass方法的Reflection类，1：main方法的所属类
    }

cc.getClassLoader()!= null 用于判断类加载器是否是bootstrap classloader。
1.Bootstrap ClassLoader - GetClassLoader的返回值为Null，是JVM的C++源码，Ext ClassLoader 的Parent，主要负责jdk_home/lib目录下的核心 api 或 -Xbootclasspath 选项指定的jar包装入工作；
2.System ClassLoader -  GetClassLoader返回值为SystemClassLoader.
主要负责java -classpath/-Djava.class.path所指的目录下的类与jar包装入工作
3.Ext ClassLoader - 是AppClassLoader 的Parent 返回Ext ClassLoader 
主要负责jdk_home/lib/ext目录下的jar包或-Djava.ext.dirs 指定目录下的jar包装入工作。
4.App ClassLoader - 返回AppClassLoader，主要负责加载用户类。
（后续在继续分析ClassLoader机制）

三：public native int getInt(Object o, long offset);注释中提到三种情况
第一种场景：The offset was obtained from {@link #objectFieldOffset} on the {@link java.lang.reflect.Field} of some Java field and the object referred to by o is of a class compatible with that field's class.

    private int offset = 10;
    public static void main(String[] args) throws Exception {
        long fieldOffset = unsafe.objectFieldOffset(Main.class.getDeclaredField("offset")) ;
        int fieldValue = unsafe.getInt(new Main(), fieldOffset);
    }

第二种场景：The offset and object reference o (either null or non-null) were both obtained via{@link #staticFieldOffset}  and {@link #staticFieldBase} (respectively) from the reflective {@link Field} representation of some Java field.

    private static int staticoffset = 20;
    public static void main(String[] args) throws Exception {
        Field staticField = Main.class.getDeclaredField("staticoffset");
        long staticFieldOffset = unsafe.staticFieldOffset(staticField);
        Object obj = unsafe.staticFieldBase(staticField);
        int staticFieldValue =unsafe.getInt(obj, staticFieldOffset);
    }

第三种场景：The object referred to by o is an array, and the offset is an integer of the form B+N*S, where N is a valid index into the array, and B and S are the values obtained by {@link #arrayBaseOffset} and {@link #arrayIndexScale} (respectively) from the array's class. The value referred to is the Nth element of the array.

    public static void main(String[] args) throws Exception {
        int [] intarray = new int[3];
        intarray[0] = 10; intarray[1] = 20; intarray[2] = 30;
        int arrayOffset = unsafe.arrayBaseOffset(intarray.getClass());
        int arrayScale = unsafe.arrayIndexScale(intarray.getClass());
        long lg = arrayOffset + 2 * arrayScale;
        int arrayValue = unsafe.getInt(intarray, lg);
    }