Java 中的 Unsafe 魔法类的作用大全

private static final Unsafe theUnsafe;

private Unsafe() {
}

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

Field theUnsafeField = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafeField.setAccessible(true);
Unsafe unsafe = (Unsafe) theUnsafeField.get(null);
System.out.println(unsafe);

//分配内存, 相当于C++的malloc函数
public native long allocateMemory(long bytes);
//扩充内存
public native long reallocateMemory(long address, long bytes);
//释放内存
public native void freeMemory(long address);
//在给定的内存块中设置值
public native void setMemory(Object o, long offset, long bytes, byte value);
//内存拷贝
public native void copyMemory(Object srcBase, long srcOffset, Object destBase, long destOffset, long bytes);
//获取给定地址值，忽略修饰限定符的访问限制。与此类似操作还有: getInt，getDouble，getLong，getChar等
public native Object getObject(Object o, long offset);
//为给定地址设置值，忽略修饰限定符的访问限制，与此类似操作还有: putInt,putDouble，putLong，putChar等
public native void putObject(Object o, long offset, Object x);
//获取给定地址的byte类型的值（当且仅当该内存地址为allocateMemory分配时，此方法结果为确定的）
public native byte getByte(long address);
//为给定地址设置byte类型的值（当且仅当该内存地址为allocateMemory分配时，此方法结果才是确定的）
public native void putByte(long address, byte x);

DirectByteBuffer(int cap) {                   // package-private

  super(-1, 0, cap, cap);
  boolean pa = VM.isDirectMemoryPageAligned();
  int ps = Bits.pageSize();
  long size = Math.max(1L, (long)cap + (pa ? ps : 0));
  Bits.reserveMemory(size, cap);

  long base = 0;
  try {
    //分配内存，返回基地址
    base = unsafe.allocateMemory(size);
  } catch (OutOfMemoryError x) {
    Bits.unreserveMemory(size, cap);
    throw x;
  }
  //内存初始化
  unsafe.setMemory(base, size, (byte) 0);
  if (pa && (base % ps != 0)) {
    // Round up to page boundary
    address = base + ps - (base & (ps - 1));
  } else {
    address = base;
  }
  //跟踪directbytebuffer 对象的垃圾回收，实现堆外内存的释放
  cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
  att = null;
}

public class Cleaner extends PhantomReference<Object> {
  
   public static Cleaner create(Object var0, Runnable var1) {
        return var1 == null ? null : add(new Cleaner(var0, var1));
    }
}

package com.rickiyang.learn.javaagent;

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.reflect.Field;


/**
 * @author rickiyang
 * @date 2019-08-08
 * @Desc
 */
public class TestPhantomReference {
  public static boolean isRun = true;

  public static void main(String[] args) throws Exception {
    String str = new String("123");
    System.out.println(str.getClass() + "@" + str.hashCode());
    final ReferenceQueue<String> referenceQueue = new ReferenceQueue<>();
    new Thread(() -> {
      while (isRun) {
        Object obj = referenceQueue.poll();
        if (obj != null) {
          try {
            Field rereferent = Reference.class.getDeclaredField("referent");
            rereferent.setAccessible(true);
            Object result = rereferent.get(obj);
            System.out.println("gc will collect："
                               + result.getClass() + "@"
                               + result.hashCode() + "\t"
                               + result);
          } catch (Exception e) {
            e.printStackTrace();
          }
        }
      }
    }).start();
    PhantomReference<String> weakRef = new PhantomReference<>(str, referenceQueue);
    str = null;
    Thread.currentThread().sleep(2000);
    System.gc();
    Thread.currentThread().sleep(2000);
    isRun = false;
  }
}

public void clean() {
  if (remove(this)) {
    try {
      this.thunk.run();
    } catch (final Throwable var2) {
      AccessController.doPrivileged(new PrivilegedAction<Void>() {
        public Void run() {
          if (System.err != null) {
            (new Error("Cleaner terminated abnormally", var2)).printStackTrace();
          }

          System.exit(1);
          return null;
        }
      });
    }

  }
}

private static class Deallocator
        implements Runnable
    {

        private static Unsafe unsafe = Unsafe.getUnsafe();

       //省略无关 代码
        public void run() {
            if (address == 0) {
                // Paranoia
                return;
            }
            unsafe.freeMemory(address);
            address = 0;
            Bits.unreserveMemory(size, capacity);
        }

    }

static {
  ThreadGroup tg = Thread.currentThread().getThreadGroup();
  for (ThreadGroup tgn = tg;
       tgn != null;
       tg = tgn, tgn = tg.getParent());
  Thread handler = new ReferenceHandler(tg, "Reference Handler");
  /* If there were a special system-only priority greater than
         * MAX_PRIORITY, it would be used here
         */
  handler.setPriority(Thread.MAX_PRIORITY);
  handler.setDaemon(true);
  handler.start();

  // provide access in SharedSecrets
  SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
    @Override
    public boolean tryHandlePendingReference() {
      return tryHandlePending(false);
    }
  });
}


static boolean tryHandlePending(boolean waitForNotify) {
  Reference<Object> r;
  Cleaner c;
  try {
    synchronized (lock) {
      if (pending != null) {
        r = pending;
        //如果当前Reference对象是Cleaner类型的就进行特殊处理
        c = r instanceof Cleaner ? (Cleaner) r : null;
        // unlink 'r' from 'pending' chain
        pending = r.discovered;
        r.discovered = null;
      } else {
        // The waiting on the lock may cause an OutOfMemoryError
        // because it may try to allocate exception objects.
        if (waitForNotify) {
          lock.wait();
        }
        // retry if waited
        return waitForNotify;
      }
    }
  } catch (OutOfMemoryError x) {
    Thread.yield();
    // retry
    return true;
  } catch (InterruptedException x) {
    // retry
    return true;
  }

  // clean 不为空的时候，走清理的逻辑
  if (c != null) {
    c.clean();
    return true;
  }

  ReferenceQueue<? super Object> q = r.queue;
  if (q != ReferenceQueue.NULL) q.enqueue(r);
  return true;
}

//读屏障，禁止load操作重排序。屏障前的load操作不能被重排序到屏障后，屏障后的load操作不能被重排序到屏障前
public native void loadFence();
//写屏障，禁止store操作重排序。屏障前的store操作不能被重排序到屏障后，屏障后的store操作不能被重排序到屏障前
public native void storeFence();
//全能屏障，禁止load、store操作重排序
public native void fullFence();

//返回系统指针的大小。返回值为4（32位系统）或 8（64位系统）。
public native int addressSize();  
//内存页的大小，此值为2的幂次方。
public native int pageSize();

private static int pageSize = -1;

static int pageSize() {
    if (pageSize == -1)
        pageSize = unsafe().pageSize();
    return pageSize;
}

// 终止挂起的线程，恢复正常.java.util.concurrent包中挂起操作都是在LockSupport类实现的，其底层正是使用这两个方法
public native void unpark(Object thread);
// 线程调用该方法，线程将一直阻塞直到超时，或者是中断条件出现。
public native void park(boolean isAbsolute, long time);
//获得对象锁（可重入锁）
@Deprecated
public native void monitorEnter(Object o);
//释放对象锁
@Deprecated
public native void monitorExit(Object o);
//尝试获取对象锁
@Deprecated
public native boolean tryMonitorEnter(Object o);

package leetcode;

import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.util.concurrent.TimeUnit;

/**
 * @author: rickiyang
 * @date: 2019/8/10
 * @description:
 */
public class TestUsafe {

    private static Thread mainThread;


    public Unsafe getUnsafe() throws Exception {
        Field theUnsafeField = Unsafe.class.getDeclaredField("theUnsafe");
        theUnsafeField.setAccessible(true);
        return (Unsafe) theUnsafeField.get(null);
    }

    public void testPark() throws Exception {
        Unsafe unsafe = getUnsafe();
        mainThread = Thread.currentThread();

        System.out.println(String.format("park %s", mainThread.getName()));
        unsafe.park(false, TimeUnit.SECONDS.toNanos(3));

        new Thread(() -> {
            System.out.println(String.format("%s unpark %s", Thread.currentThread().getName(),
                                             mainThread.getName()));
            unsafe.unpark(mainThread);
        }).start();
        System.out.println("main thread is done");

    }

    public static void main(String[] args) throws Exception {
        TestUsafe testUsafe = new TestUsafe();
        testUsafe.testPark();
    }

}

// 传入一个Class对象并创建该实例对象，但不会调用构造方法
public native Object allocateInstance(Class<?> cls) throws InstantiationException;

// 获取字段f在实例对象中的偏移量
public native long objectFieldOffset(Field f);

// 返回值就是f.getDeclaringClass()
public native Object staticFieldBase(Field f);
// 静态属性的偏移量，用于在对应的Class对象中读写静态属性
public native long staticFieldOffset(Field f);

// 获得给定对象偏移量上的int值，所谓的偏移量可以简单理解为指针指向该变量；的内存地址，
// 通过偏移量便可得到该对象的变量，进行各种操作
public native int getInt(Object o, long offset);
// 设置给定对象上偏移量的int值
public native void putInt(Object o, long offset, int x);

// 获得给定对象偏移量上的引用类型的值
public native Object getObject(Object o, long offset);
// 设置给定对象偏移量上的引用类型的值
public native void putObject(Object o, long offset, Object x););

// 设置给定对象的int值，使用volatile语义，即设置后立马更新到内存对其他线程可见
public native void putIntVolatile(Object o, long offset, int x);
// 获得给定对象的指定偏移量offset的int值，使用volatile语义，总能获取到最新的int值。
public native int getIntVolatile(Object o, long offset);

// 与putIntVolatile一样，但要求被操作字段必须有volatile修饰
public native void putOrderedInt(Object o, long offset, int x);

public class User {

    private String name;
    private int age;
    private static String address = "beijing";

    public User(){
        name = "xiaoming";
    }

    public String getname(){
        return name;
    }
}

	/**
     * 实例化对象
     * @throws Exception
     */
public void newInstance() throws Exception{
    TestUsafe testUsafe = new TestUsafe();
    Unsafe unsafe = testUsafe.getUnsafe();
    User user = new User();
    System.out.println(user.getname());

    User user1 = User.class.newInstance();
    System.out.println(user1.getname());

    User o = (User)unsafe.allocateInstance(User.class);
    System.out.println(o.getname());
}

public void testObject() throws Exception{
    TestUsafe testUsafe = new TestUsafe();
    Unsafe unsafe = testUsafe.getUnsafe();

    //通过allocateInstance创建对象,为其分配内存地址，不会加载构造函数
    User user = (User) unsafe.allocateInstance(User.class);
    System.out.println(user);

    // Class && Field
    Class<? extends User> userClass = user.getClass();
    Field name = userClass.getDeclaredField("name");
    Field age = userClass.getDeclaredField("age");
    Field location = userClass.getDeclaredField("address");

    // 获取实例域name和age在对象内存中的偏移量并设置值
    System.out.println(unsafe.objectFieldOffset(name));
    unsafe.putObject(user, unsafe.objectFieldOffset(name), "xiaoming");
    System.out.println(unsafe.objectFieldOffset(age));
    unsafe.putInt(user, unsafe.objectFieldOffset(age), 18);
    System.out.println(user);

    // 获取定义location字段的类
    Object staticFieldBase = unsafe.staticFieldBase(location);
    System.out.println(staticFieldBase);

    // 获取static变量address的偏移量
    long staticFieldOffset = unsafe.staticFieldOffset(location);
    // 获取static变量address的值
    System.out.println(unsafe.getObject(staticFieldBase, staticFieldOffset));
    // 设置static变量address的值
    unsafe.putObject(staticFieldBase, staticFieldOffset, "tianjin");
    System.out.println(user + " " + user.getAddress());
}

public class User {

    private String name;
    private int age;
}

TestUsafe testUsafe = new TestUsafe();
Unsafe unsafe = testUsafe.getUnsafe();

for (Field field : User.class.getDeclaredFields()) {
    System.out.println(field.getName() + "-" + field.getType() + ": " + unsafe.objectFieldOffset(field));
}

结果：
name-class java.lang.String: 16
age-int: 12

public static long sizeOf(Object o) throws Exception{
    TestUsafe testUsafe = new TestUsafe();
    Unsafe unsafe = testUsafe.getUnsafe();
    HashSet<Field> fields = new HashSet<Field>();
    Class c = o.getClass();
    while (c != Object.class) {
        for (Field f : c.getDeclaredFields()) {
            if ((f.getModifiers() & Modifier.STATIC) == 0) {
                fields.add(f);
            }
        }
        //如果有继承父类的话，父类中的属性也是要计算的
        c = c.getSuperclass();
    }
    //计算每个字段的偏移量，因为第一个字段的偏移量即在对象头的基础上偏移的
    //所以只需要比较当前偏移量最大的字段即表示这是该对象最后一个字段的位置
    long maxSize = 0;
    for (Field f : fields) {
        long offset = unsafe.objectFieldOffset(f);
        if (offset > maxSize) {
            maxSize = offset;
        }
    }
    //上面计算的是对象最后一个字段的偏移量起始位置，java中对象最大长度是8个字节(long)
    //这里的计算方式是 将 当前偏移量 / 8 + 8字节 的padding
    return ((maxSize/8) + 1) * 8;
}

//静态属性的偏移量，用于在对应的Class对象中读写静态属性
public native long staticFieldOffset(Field f);
//获取一个静态字段的对象指针
public native Object staticFieldBase(Field f);
//判断是否需要初始化一个类，通常在获取一个类的静态属性的时候（因为一个类如果没初始化，它的静态属性也不会初始化）使用。 当且仅当ensureClassInitialized方法不生效时返回false
public native boolean shouldBeInitialized(Class<?> c);
//确保类被初始化
public native void ensureClassInitialized(Class<?> c);
//定义一个类，可用于动态创建类，此方法会跳过JVM的所有安全检查，默认情况下，ClassLoader（类加载器）和ProtectionDomain（保护域）实例来源于调用者
public native Class<?> defineClass(String name, byte[] b, int off, int len,
                                   ClassLoader loader,
                                   ProtectionDomain protectionDomain);
//定义一个匿名类，可用于动态创建类
public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches);

//返回数组中第一个元素的偏移地址
public native int arrayBaseOffset(Class<?> arrayClass);
//返回数组中一个元素占用的大小
public native int arrayIndexScale(Class<?> arrayClass);

public final native boolean compareAndSwapObject(Object o, long offset,  Object expected, Object update);

public final native boolean compareAndSwapInt(Object o, long offset, int expected,int update);
  
public final native boolean compareAndSwapLong(Object o, long offset, long expected, long update);

//1.8新增，给定对象o，根据获取内存偏移量指向的字段，将其增加delta，
 //这是一个CAS操作过程，直到设置成功方能退出循环，返回旧值
 public final int getAndAddInt(Object o, long offset, int delta) {
     int v;
     do {
         //获取内存中最新值
         v = getIntVolatile(o, offset);
       //通过CAS操作
     } while (!compareAndSwapInt(o, offset, v, v + delta));
     return v;
 }

//1.8新增，方法作用同上，只不过这里操作的long类型数据
 public final long getAndAddLong(Object o, long offset, long delta) {
     long v;
     do {
         v = getLongVolatile(o, offset);
     } while (!compareAndSwapLong(o, offset, v, v + delta));
     return v;
 }

 //1.8新增，给定对象o，根据获取内存偏移量对于字段，将其 设置为新值newValue，
 //这是一个CAS操作过程，直到设置成功方能退出循环，返回旧值
 public final int getAndSetInt(Object o, long offset, int newValue) {
     int v;
     do {
         v = getIntVolatile(o, offset);
     } while (!compareAndSwapInt(o, offset, v, newValue));
     return v;
 }

// 1.8新增，同上，操作的是long类型
 public final long getAndSetLong(Object o, long offset, long newValue) {
     long v;
     do {
         v = getLongVolatile(o, offset);
     } while (!compareAndSwapLong(o, offset, v, newValue));
     return v;
 }

 //1.8新增，同上，操作的是引用类型数据
 public final Object getAndSetObject(Object o, long offset, Object newValue) {
     Object v;
     do {
         v = getObjectVolatile(o, offset);
     } while (!compareAndSwapObject(o, offset, v, newValue));
     return v;
 }