【Java基础】反射-UnSafe

柔情只为你懂 2022-09-04 05:48 139阅读 0赞

## 如何正确创建UnSafe实例 ##

Long offset = Unsafe.getUnsafe().objectFieldOffset(nameField);
    unsafe.compareAndSwapObject(targetInstance,offset,name,"hello unsafe");

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查看getUnsafe方法，会判断当前类的class loader是否为null，但当前类的加载器是AppClassLoader实例，所以这里判断为真，抛出上面的异常。所以只能用BootstrapClassLoader加载器中调用的类使用这个unsage，比如concurrenthashmap是在BootstrapClassLoader中加载的，getClassLoader0()获取出来是为null的，所以不会抛出异常。

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

public static boolean isSystemDomainLoader(ClassLoader var0) { 
        return var0 == null;
    }

那如何得到UnSafe的实例呢？因为Unsafe中有个theUnsafe的属性，该属性在static静态代码块中被实例化类，所以可以通过放射来获取这个已经实例化的对象属性，从而获得unsafe对象：

static { 
        registerNatives();
        Reflection.registerMethodsToFilter(Unsafe.class, new String[]{ "getUnsafe"});
        theUnsafe = new Unsafe();
        ...
        }

## 如何使用UnSafe进行反射？ ##

创建一个反射的目标类用于后面的例子

public class Hello { 
    
        private String name;
    
        public void hello(String name){ 
            System.out.println("hello "+name);
        }
    
        public void say(String name){ 
            System.out.println("say "+name);
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        public String getName() { 
            return name;
        }
    
    }

通过UnSafe操作目标对象的属性

//通过反射拿到Unsafe类的成员theUnsafe，该成员已经在UnSafe类的静态代码块中被实例化了
    Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
    theUnsafe.setAccessible(true);
    Unsafe unsafe = (Unsafe) theUnsafe.get(null);
    
    Class clazz = Hello.class;
    Field nameField = clazz.getDeclaredField("name");
    nameField.setAccessible(true);
    
    Hello targetInstance = new Hello();
    //这里可以使用反射拿到成员变量theUnsafe
    Long offset = unsafe.objectFieldOffset(nameField);
    unsafe.compareAndSwapObject(targetInstance,offset,name,"hello unsafe");
    String name = (String) unsafe.getObject(targetInstance,offset);
    System.out.println("name: "+name);

## 结语 ##

当然UnSafe还有内存操作、内存屏障、线程调度等功能，后续会有专题文章。