Java Unsafe学习笔记分享

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

//方法一：我们可以令我们的代码“受信任”。运行程序时，使用bootclasspath选项，指定系统类路径加上你使用的一个Unsafe路径
java -Xbootclasspath:/usr/jdk1.7.0/jre/lib/rt.jar:. com.mishadoff.magic.UnsafeClient
// 方法二
static {
    try {
        Field field = Unsafe.class.getDeclaredField("theUnsafe");
        field.setAccessible(true);
        UNSAFE = (Unsafe) field.get(null);
    } catch (Exception e) {
    }
}

User instance = (User) UNSAFE.allocateInstance(User.class);

package com.quancheng;
import sun.misc.Unsafe;
import java.lang.reflect.Field;
public class CollectionApp {
    private static sun.misc.Unsafe UNSAFE;
    public static void main(String[] args) {
        try {
            User instance = (User) UNSAFE.allocateInstance(User.class);
            instance.setName("luoyoub");
            System.err.println("instance:" + instance);
            instance.test();
            Field name = instance.getClass().getDeclaredField("name");
            UNSAFE.putObject(instance, UNSAFE.objectFieldOffset(name), "huanghui");
            instance.test();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    static {
        try {
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            UNSAFE = (Unsafe) field.get(null);
        } catch (Exception e) {
        }
    }
}
class User {
    private String name;
    public void setName(String name) {
        this.name = name;
    }
    public void test() {
        System.err.println("hello,world" + name);
    }
}

public class CollectionApp {
    private static sun.misc.Unsafe UNSAFE;
    public static void main(String[] args) throws IllegalAccessException, InstantiationException {
        A a = new A();
        a.test(); // output ==> 1
        A a1 = A.class.newInstance();
        a1.test(); // output ==> 1
        A instance = (A) UNSAFE.allocateInstance(A.class);
        instance.test(); // output ==> 0
    }
    static {
        try {
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            UNSAFE = (Unsafe) field.get(null);
        } catch (Exception e) {
        }
    }
}
class A{
    private long a;
    public A(){
        a = 1;
    }
    public void test(){
        System.err.println("a==>" + a);
    }
}

Guard guard = new Guard();
guard.giveAccess();   // false, no access
// bypass
Unsafe unsafe = getUnsafe();
Field f = guard.getClass().getDeclaredField("ACCESS_ALLOWED");
unsafe.putInt(guard, unsafe.objectFieldOffset(f), 42); // memory corruption
guard.giveAccess(); // true, access granted

private static byte[] getClassContent() throws Exception {
    File f = new File("/home/mishadoff/tmp/A.class");
    FileInputStream input = new FileInputStream(f);
    byte[] content = new byte[(int)f.length()];
    input.read(content);
    input.close();
    return content;
}
byte[] classContents = getClassContent();
Class c = getUnsafe().defineClass(
              null, classContents, 0, classContents.length);
c.getMethod("a").invoke(c.newInstance(), null); // 1

getUnsafe().throwException(new IOException());

class SuperArray {
    private final static int BYTE = 1;
    private long size;
    private long address;
    public SuperArray(long size) {
        this.size = size;
        address = getUnsafe().allocateMemory(size * BYTE);
    }
    public void set(long i, byte value) {
        getUnsafe().putByte(address + i * BYTE, value);
    }
    public int get(long idx) {
        return getUnsafe().getByte(address + idx * BYTE);
    }
    public long size() {
        return size;
    }
}
long SUPER_SIZE = (long)Integer.MAX_VALUE * 2;
SuperArray array = new SuperArray(SUPER_SIZE);
System.out.println("Array size:" + array.size()); // 4294967294
for (int i = 0; i < 100; i++) {
    array.set((long)Integer.MAX_VALUE + i, (byte)3);
    sum += array.get((long)Integer.MAX_VALUE + i);
}
System.out.println("Sum of 100 elements:" + sum);  // 300

public native void unpark(Thread jthread);  
public native void park(boolean isAbsolute, long time); // isAbsolute参数是指明时间是绝对的，还是相对的

Example1:
// 针对当前线程已经调用过unpark(多次调用unpark的效果和调用一次unpark的效果一样)
public static void main(String[] args) throws InterruptedException {
    Thread currThread = Thread.currentThread();
    UNSAFE.unpark(currThread);
    UNSAFE.unpark(currThread);
    UNSAFE.unpark(currThread);
    UNSAFE.park(false, 0);
    UNSAFE.park(false, 0);
    System.out.println("SUCCESS!!!");
}
// 恢复线程interrupt() && UNSAFE.unpark()运行结果一样
public static void main(String[] args) throws InterruptedException {
    Thread currThread = Thread.currentThread();
    new Thread(()->{
        try {
            Thread.sleep(3000);
            System.err.println("sub thread end");
            // currThread.interrupt();
            UNSAFE.unpark(currThread);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }).start();
    UNSAFE.park(false, 0);
    System.out.println("SUCCESS!!!");
}
// 如果是相对时间也就是isAbsolute为false（注意这里后面的单位纳秒）到期的时候，与Thread.sleep效果相同，具体有什么区别有待深入研究
//相对时间后面的参数单位是纳秒
UNSAFE.park(false, 3000000000l);
System.out.println("SUCCESS!!!");
long time = System.currentTimeMillis()+3000;
//绝对时间后面的参数单位是毫秒
UNSAFE.park(true, time);
System.out.println("SUCCESS!!!");

sun.misc.Unsafe类包含105个方法。实际上，对各种实体操作有几组重要方法，其中的一些如下：
Info.仅返回一些低级的内存信息
addressSize
pageSize
Objects.提供用于操作对象及其字段的方法
allocateInstance     ##直接获取对象实例
objectFieldOffset
Classes.提供用于操作类及其静态字段的方法
staticFieldOffset
defineClass
defineAnonymousClass
ensureClassInitialized
Arrays.操作数组
arrayBaseOffset
arrayIndexScale
Synchronization.低级的同步原语
monitorEnter
tryMonitorEnter
monitorExit
compareAndSwapInt
putOrderedInt
Memory.直接内存访问方法
allocateMemory
copyMemory
freeMemory
getAddress
getInt
putInt

// Thread.sleep会抛异常
public static void main(String[] args) throws InterruptedException {
   Thread thread =  new Thread(()->{
        try {
            System.err.println("sub thread start");
            Thread.sleep(10000);
            System.err.println("sub thread end");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    });
   thread.start();
    TimeUnit.SECONDS.sleep(3);
    thread.interrupt();
    System.out.println("SUCCESS!!!");
}
output==>
sub thread start
SUCCESS!!!
java.lang.InterruptedException: sleep interrupted
    at java.lang.Thread.sleep(Native Method)
    at com.quancheng.ConcurrentTest.lambda$main$0(ConcurrentTest.java:13)
    at java.lang.Thread.run(Thread.java:745)
Process finished with exit code 0
public static void main(String[] args) throws InterruptedException {
   Thread thread =  new Thread(()->{
       System.err.println("sub thread start");
       UNSAFE.park(false,0);
       System.err.println("sub thread end");
    });
   thread.start();
    TimeUnit.SECONDS.sleep(3);
    UNSAFE.unpark(thread);
    System.out.println("SUCCESS!!!");
}
output==>
sub thread start
sub thread end
SUCCESS!!!
Process finished with exit code 0

public static void main(String[] args) throws InterruptedException {
    Thread thread = new Thread(() -> {
        try {
            System.err.println("sub thread start");
            TimeUnit.SECONDS.sleep(10);
            System.err.println("sub thread end");
        } catch (Exception e) {
            e.printStackTrace();
        }
    });
    thread.start();
    TimeUnit.SECONDS.sleep(3);
    UNSAFE.unpark(thread);
    System.out.println("SUCCESS!!!");
}