Java 中的 Unsafe 魔法類的作用大全

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);

//分配內存, 相當於C++的malloc函數
public native long allocateMemory(long bytes);
//擴充內存
public native long reallocateMemory(long address, long bytes);
//釋放內存
public native void freeMemory(long address);
//在給定的內存塊中設置值
public native void setMemory(Object o, long offset, long bytes, byte value);
//內存拷貝
public native void copyMemory(Object srcBase, long srcOffset, Object destBase, long destOffset, long bytes);
//獲取給定地址值，忽略修飾限定符的訪問限制。與此類似操作還有: getInt，getDouble，getLong，getChar等
public native Object getObject(Object o, long offset);
//為給定地址設置值，忽略修飾限定符的訪問限制，與此類似操作還有: putInt,putDouble，putLong，putChar等
public native void putObject(Object o, long offset, Object x);
//獲取給定地址的byte類型的值（當且僅當該內存地址為allocateMemory分配時，此方法結果為確定的）
public native byte getByte(long address);
//為給定地址設置byte類型的值（當且僅當該內存地址為allocateMemory分配時，此方法結果才是確定的）
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 {
    //分配內存，返回基地址
    base = unsafe.allocateMemory(size);
  } catch (OutOfMemoryError x) {
    Bits.unreserveMemory(size, cap);
    throw x;
  }
  //內存初始化
  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;
  }
  //跟蹤directbytebuffer 對象的垃圾回收，實現堆外內存的釋放
  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();

       //省略無關 代碼
        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;
        //如果當前Reference對象是Cleaner類型的就進行特殊處理
        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 不為空的時候，走清理的邏輯
  if (c != null) {
    c.clean();
    return true;
  }

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

//讀屏障，禁止load操作重排序。屏障前的load操作不能被重排序到屏障後，屏障後的load操作不能被重排序到屏障前
public native void loadFence();
//寫屏障，禁止store操作重排序。屏障前的store操作不能被重排序到屏障後，屏障後的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();

//返回系統指針的大小。返回值為4（32位系統）或 8（64位系統）。
public native int addressSize();  
//內存頁的大小，此值為2的冪次方。
public native int pageSize();

private static int pageSize = -1;

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

// 終止掛起的線程，恢復正常.java.util.concurrent包中掛起操作都是在LockSupport類實現的，其底層正是使用這兩個方法
public native void unpark(Object thread);
// 線程調用該方法，線程將一直阻塞直到超時，或者是中斷條件出現。
public native void park(boolean isAbsolute, long time);
//獲得對象鎖（可重入鎖）
@Deprecated
public native void monitorEnter(Object o);
//釋放對象鎖
@Deprecated
public native void monitorExit(Object o);
//嘗試獲取對象鎖
@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();
    }

}

// 傳入一個Class對象並創建該實例對象，但不會調用構造方法
public native Object allocateInstance(Class<?> cls) throws InstantiationException;

// 獲取字段f在實例對象中的偏移量
public native long objectFieldOffset(Field f);

// 返回值就是f.getDeclaringClass()
public native Object staticFieldBase(Field f);
// 靜態屬性的偏移量，用於在對應的Class對象中讀寫靜態屬性
public native long staticFieldOffset(Field f);

// 獲得給定對象偏移量上的int值，所謂的偏移量可以簡單理解為指針指向該變量；的內存地址，
// 通過偏移量便可得到該對象的變量，進行各種操作
public native int getInt(Object o, long offset);
// 設置給定對象上偏移量的int值
public native void putInt(Object o, long offset, int x);

// 獲得給定對象偏移量上的引用類型的值
public native Object getObject(Object o, long offset);
// 設置給定對象偏移量上的引用類型的值
public native void putObject(Object o, long offset, Object x););

// 設置給定對象的int值，使用volatile語義，即設置後立馬更新到內存對其他線程可見
public native void putIntVolatile(Object o, long offset, int x);
// 獲得給定對象的指定偏移量offset的int值，使用volatile語義，總能獲取到最新的int值。
public native int getIntVolatile(Object o, long offset);

// 與putIntVolatile一樣，但要求被操作字段必須有volatile修飾
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;
    }
}

	/**
     * 實例化對象
     * @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();

    //通過allocateInstance創建對象,為其分配內存地址，不會加載構造函數
    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");

    // 獲取實例域name和age在對象內存中的偏移量並設置值
    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);

    // 獲取定義location字段的類
    Object staticFieldBase = unsafe.staticFieldBase(location);
    System.out.println(staticFieldBase);

    // 獲取static變量address的偏移量
    long staticFieldOffset = unsafe.staticFieldOffset(location);
    // 獲取static變量address的值
    System.out.println(unsafe.getObject(staticFieldBase, staticFieldOffset));
    // 設置static變量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));
}

結果：
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);
            }
        }
        //如果有繼承父類的話，父類中的屬性也是要計算的
        c = c.getSuperclass();
    }
    //計算每個字段的偏移量，因為第一個字段的偏移量即在對象頭的基礎上偏移的
    //所以隻需要比較當前偏移量最大的字段即表示這是該對象最後一個字段的位置
    long maxSize = 0;
    for (Field f : fields) {
        long offset = unsafe.objectFieldOffset(f);
        if (offset > maxSize) {
            maxSize = offset;
        }
    }
    //上面計算的是對象最後一個字段的偏移量起始位置，java中對象最大長度是8個字節(long)
    //這裡的計算方式是 將 當前偏移量 / 8 + 8字節 的padding
    return ((maxSize/8) + 1) * 8;
}

//靜態屬性的偏移量，用於在對應的Class對象中讀寫靜態屬性
public native long staticFieldOffset(Field f);
//獲取一個靜態字段的對象指針
public native Object staticFieldBase(Field f);
//判斷是否需要初始化一個類，通常在獲取一個類的靜態屬性的時候（因為一個類如果沒初始化，它的靜態屬性也不會初始化）使用。 當且僅當ensureClassInitialized方法不生效時返回false
public native boolean shouldBeInitialized(Class<?> c);
//確保類被初始化
public native void ensureClassInitialized(Class<?> c);
//定義一個類，可用於動態創建類，此方法會跳過JVM的所有安全檢查，默認情況下，ClassLoader（類加載器）和ProtectionDomain（保護域）實例來源於調用者
public native Class<?> defineClass(String name, byte[] b, int off, int len,
                                   ClassLoader loader,
                                   ProtectionDomain protectionDomain);
//定義一個匿名類，可用於動態創建類
public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches);

//返回數組中第一個元素的偏移地址
public native int arrayBaseOffset(Class<?> arrayClass);
//返回數組中一個元素占用的大小
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，給定對象o，根據獲取內存偏移量指向的字段，將其增加delta，
 //這是一個CAS操作過程，直到設置成功方能退出循環，返回舊值
 public final int getAndAddInt(Object o, long offset, int delta) {
     int v;
     do {
         //獲取內存中最新值
         v = getIntVolatile(o, offset);
       //通過CAS操作
     } while (!compareAndSwapInt(o, offset, v, v + delta));
     return v;
 }

//1.8新增，方法作用同上，隻不過這裡操作的long類型數據
 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，給定對象o，根據獲取內存偏移量對於字段，將其 設置為新值newValue，
 //這是一個CAS操作過程，直到設置成功方能退出循環，返回舊值
 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類型
 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，操作的是引用類型數據
 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;
 }