Unsafe API说明

电玩女神 2022-08-10 09:22 178阅读 0赞

package sun.misc;
    import java.lang.reflect.Field;
    /***
     * This class should provide access to low-level operations and its
     * use should be limited to trusted code.  Fields can be accessed using
     * memory addresses, with undefined behaviour occurring if invalid memory
     * addresses are given.
     * 这个类提供了一个更底层的操作并且应该在受信任的代码中使用。可以通过内存地址
     * 存取fields,如果给出的内存地址是无效的那么会有一个不确定的运行表现。
     * 
     * @author Tom Tromey (tromey@redhat.com)
     * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
     */
    public class Unsafe
    {
      // Singleton class.
      private static Unsafe unsafe = new Unsafe();
      /***
       * Private default constructor to prevent creation of an arbitrary
       * number of instances.
       * 使用私有默认构造器防止创建多个实例
       */
      private Unsafe()
      {
      }
      /***
       * Retrieve the singleton instance of <code>Unsafe</code>.  The calling
       * method should guard this instance from untrusted code, as it provides
       * access to low-level operations such as direct memory access.
       * 获取<code>Unsafe</code>的单例,这个方法调用应该防止在不可信的代码中实例，
       * 因为unsafe类提供了一个低级别的操作，例如直接内存存取。
       * 
       * @throws SecurityException if a security manager exists and prevents
       *                           access to the system properties.
       *                           如果安全管理器不存在或者禁止访问系统属性
       */
      public static Unsafe getUnsafe()
      {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null)
          sm.checkPropertiesAccess();
        return unsafe;
      }
      
      /***
       * Returns the memory address offset of the given static field.
       * The offset is merely used as a means to access a particular field
       * in the other methods of this class.  The value is unique to the given
       * field and the same value should be returned on each subsequent call.
       * 返回指定静态field的内存地址偏移量,在这个类的其他方法中这个值只是被用作一个访问
       * 特定field的一个方式。这个值对于 给定的field是唯一的，并且后续对该方法的调用都应该
       * 返回相同的值。
       *
       * @param field the field whose offset should be returned.
       *              需要返回偏移量的field
       * @return the offset of the given field.
       *         指定field的偏移量
       */
      public native long objectFieldOffset(Field field);
      /***
       * Compares the value of the integer field at the specified offset
       * in the supplied object with the given expected value, and updates
       * it if they match.  The operation of this method should be atomic,
       * thus providing an uninterruptible way of updating an integer field.
       * 在obj的offset位置比较integer field和期望的值，如果相同则更新。这个方法
       * 的操作应该是原子的，因此提供了一种不可中断的方式更新integer field。
       * 
       * @param obj the object containing the field to modify.
       *            包含要修改field的对象
       * @param offset the offset of the integer field within <code>obj</code>.
       *               <code>obj</code>中整型field的偏移量
       * @param expect the expected value of the field.
       *               希望field中存在的值
       * @param update the new value of the field if it equals <code>expect</code>.
       *           如果期望值expect与field的当前值相同，设置filed的值为这个新值
       * @return true if the field was changed.
       *                             如果field的值被更改
       */
      public native boolean compareAndSwapInt(Object obj, long offset,
                                              int expect, int update);
      /***
       * Compares the value of the long field at the specified offset
       * in the supplied object with the given expected value, and updates
       * it if they match.  The operation of this method should be atomic,
       * thus providing an uninterruptible way of updating a long field.
       * 在obj的offset位置比较long field和期望的值，如果相同则更新。这个方法
       * 的操作应该是原子的，因此提供了一种不可中断的方式更新long field。
       * 
       * @param obj the object containing the field to modify.
       *              包含要修改field的对象 
       * @param offset the offset of the long field within <code>obj</code>.
       *               <code>obj</code>中long型field的偏移量
       * @param expect the expected value of the field.
       *               希望field中存在的值
       * @param update the new value of the field if it equals <code>expect</code>.
       *               如果期望值expect与field的当前值相同，设置filed的值为这个新值
       * @return true if the field was changed.
       *              如果field的值被更改
       */
      public native boolean compareAndSwapLong(Object obj, long offset,
                                               long expect, long update);
      /***
       * Compares the value of the object field at the specified offset
       * in the supplied object with the given expected value, and updates
       * it if they match.  The operation of this method should be atomic,
       * thus providing an uninterruptible way of updating an object field.
       * 在obj的offset位置比较object field和期望的值，如果相同则更新。这个方法
       * 的操作应该是原子的，因此提供了一种不可中断的方式更新object field。
       * 
       * @param obj the object containing the field to modify.
       *    包含要修改field的对象 
       * @param offset the offset of the object field within <code>obj</code>.
       *         <code>obj</code>中object型field的偏移量
       * @param expect the expected value of the field.
       *               希望field中存在的值
       * @param update the new value of the field if it equals <code>expect</code>.
       *               如果期望值expect与field的当前值相同，设置filed的值为这个新值
       * @return true if the field was changed.
       *              如果field的值被更改
       */
      public native boolean compareAndSwapObject(Object obj, long offset,
                                                 Object expect, Object update);
      /***
       * Sets the value of the integer field at the specified offset in the
       * supplied object to the given value.  This is an ordered or lazy
       * version of <code>putIntVolatile(Object,long,int)</code>, which
       * doesn't guarantee the immediate visibility of the change to other
       * threads.  It is only really useful where the integer field is
       * <code>volatile</code>, and is thus expected to change unexpectedly.
       * 设置obj对象中offset偏移地址对应的整型field的值为指定值。这是一个有序或者
       * 有延迟的<code>putIntVolatile</cdoe>方法，并且不保证值的改变被其他线程立
       * 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候
       * 使用才有用。
       * 
       * @param obj the object containing the field to modify.
       *    包含需要修改field的对象
       * @param offset the offset of the integer field within <code>obj</code>.
       *       <code>obj</code>中整型field的偏移量
       * @param value the new value of the field.
       *      field将被设置的新值
       * @see #putIntVolatile(Object,long,int)
       */
      public native void putOrderedInt(Object obj, long offset, int value);
      /***
       * Sets the value of the long field at the specified offset in the
       * supplied object to the given value.  This is an ordered or lazy
       * version of <code>putLongVolatile(Object,long,long)</code>, which
       * doesn't guarantee the immediate visibility of the change to other
       * threads.  It is only really useful where the long field is
       * <code>volatile</code>, and is thus expected to change unexpectedly.
       * 设置obj对象中offset偏移地址对应的long型field的值为指定值。这是一个有序或者
       * 有延迟的<code>putLongVolatile</cdoe>方法，并且不保证值的改变被其他线程立
       * 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候
       * 使用才有用。
       * 
       * @param obj the object containing the field to modify.
       *    包含需要修改field的对象
       * @param offset the offset of the long field within <code>obj</code>.
       *       <code>obj</code>中long型field的偏移量
       * @param value the new value of the field.
       *      field将被设置的新值
       * @see #putLongVolatile(Object,long,long)
       */
      public native void putOrderedLong(Object obj, long offset, long value);
      /***
       * Sets the value of the object field at the specified offset in the
       * supplied object to the given value.  This is an ordered or lazy
       * version of <code>putObjectVolatile(Object,long,Object)</code>, which
       * doesn't guarantee the immediate visibility of the change to other
       * threads.  It is only really useful where the object field is
       * <code>volatile</code>, and is thus expected to change unexpectedly.
       * 设置obj对象中offset偏移地址对应的object型field的值为指定值。这是一个有序或者
       * 有延迟的<code>putObjectVolatile</cdoe>方法，并且不保证值的改变被其他线程立
       * 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候
       * 使用才有用。
       *
       * @param obj the object containing the field to modify.
       *    包含需要修改field的对象
       * @param offset the offset of the object field within <code>obj</code>.
       *       <code>obj</code>中long型field的偏移量
       * @param value the new value of the field.
       *      field将被设置的新值
       */
      public native void putOrderedObject(Object obj, long offset, Object value);
      /***
       * Sets the value of the integer field at the specified offset in the
       * supplied object to the given value, with volatile store semantics.
       * 设置obj对象中offset偏移地址对应的整型field的值为指定值。支持volatile store语义
       * 
       * @param obj the object containing the field to modify.
       *    包含需要修改field的对象
       * @param offset the offset of the integer field within <code>obj</code>.
       *       <code>obj</code>中整型field的偏移量
       * @param value the new value of the field.
       *       field将被设置的新值
       */
      public native void putIntVolatile(Object obj, long offset, int value);
      /***
       * Retrieves the value of the integer field at the specified offset in the
       * supplied object with volatile load semantics.
       * 获取obj对象中offset偏移地址对应的整型field的值,支持volatile load语义。
       * 
       * @param obj the object containing the field to read.
       *    包含需要去读取的field的对象
       * @param offset the offset of the integer field within <code>obj</code>.
       *       <code>obj</code>中整型field的偏移量
       */
      public native int getIntVolatile(Object obj, long offset);
      /***
       * Sets the value of the long field at the specified offset in the
       * supplied object to the given value, with volatile store semantics.
       * 设置obj对象中offset偏移地址对应的long型field的值为指定值。支持volatile store语义
       *
       * @param obj the object containing the field to modify.
       *            包含需要修改field的对象
       * @param offset the offset of the long field within <code>obj</code>.
       *               <code>obj</code>中long型field的偏移量
       * @param value the new value of the field.
       *              field将被设置的新值
       * @see #putLong(Object,long,long)
       */
      public native void putLongVolatile(Object obj, long offset, long value);
      /***
       * Sets the value of the long field at the specified offset in the
       * supplied object to the given value.
       * 设置obj对象中offset偏移地址对应的long型field的值为指定值。
       * 
       * @param obj the object containing the field to modify.
       *     包含需要修改field的对象
       * @param offset the offset of the long field within <code>obj</code>.
       *     <code>obj</code>中long型field的偏移量
       * @param value the new value of the field.
       *     field将被设置的新值
       * @see #putLongVolatile(Object,long,long)
       */
      public native void putLong(Object obj, long offset, long value);
      /***
       * Retrieves the value of the long field at the specified offset in the
       * supplied object with volatile load semantics.
       * 获取obj对象中offset偏移地址对应的long型field的值,支持volatile load语义。
       * 
       * @param obj the object containing the field to read.
       *    包含需要去读取的field的对象
       * @param offset the offset of the long field within <code>obj</code>.
       *       <code>obj</code>中long型field的偏移量
       * @see #getLong(Object,long)
       */
      public native long getLongVolatile(Object obj, long offset);
      /***
       * Retrieves the value of the long field at the specified offset in the
       * supplied object.
       * 获取obj对象中offset偏移地址对应的long型field的值
       * 
       * @param obj the object containing the field to read.
       *    包含需要去读取的field的对象
       * @param offset the offset of the long field within <code>obj</code>.
       *       <code>obj</code>中long型field的偏移量
       * @see #getLongVolatile(Object,long)
       */
      public native long getLong(Object obj, long offset);
      /***
       * Sets the value of the object field at the specified offset in the
       * supplied object to the given value, with volatile store semantics.
       * 设置obj对象中offset偏移地址对应的object型field的值为指定值。支持volatile store语义
       * 
       * @param obj the object containing the field to modify.
       *    包含需要修改field的对象
       * @param offset the offset of the object field within <code>obj</code>.
       *     <code>obj</code>中object型field的偏移量
       * @param value the new value of the field.
       *       field将被设置的新值
       * @see #putObject(Object,long,Object)
       */
      public native void putObjectVolatile(Object obj, long offset, Object value);
      /***
       * Sets the value of the object field at the specified offset in the
       * supplied object to the given value.
       * 设置obj对象中offset偏移地址对应的object型field的值为指定值。
       * 
       * @param obj the object containing the field to modify.
       *    包含需要修改field的对象
       * @param offset the offset of the object field within <code>obj</code>.
       *     <code>obj</code>中object型field的偏移量
       * @param value the new value of the field.
       *       field将被设置的新值
       * @see #putObjectVolatile(Object,long,Object)
       */
      public native void putObject(Object obj, long offset, Object value);
      /***
       * Retrieves the value of the object field at the specified offset in the
       * supplied object with volatile load semantics.
       * 获取obj对象中offset偏移地址对应的object型field的值,支持volatile load语义。
       * 
       * @param obj the object containing the field to read.
       *    包含需要去读取的field的对象
       * @param offset the offset of the object field within <code>obj</code>.
       *       <code>obj</code>中object型field的偏移量
       */
      public native Object getObjectVolatile(Object obj, long offset);
      /***
       * Returns the offset of the first element for a given array class.
       * To access elements of the array class, this value may be used along with
       * with that returned by 
       * <a href="#arrayIndexScale"><code>arrayIndexScale</code></a>,
       * if non-zero.
       * 获取给定数组中第一个元素的偏移地址。
       * 为了存取数组中的元素，这个偏移地址与<a href="#arrayIndexScale"><code>arrayIndexScale
       * </code></a>方法的非0返回值一起被使用。
       * @param arrayClass the class for which the first element's address should
       *                   be obtained.
       *                   第一个元素地址被获取的class
       * @return the offset of the first element of the array class.
       *    数组第一个元素 的偏移地址
       * @see arrayIndexScale(Class)
       */
      public native int arrayBaseOffset(Class arrayClass);
      /***
       * Returns the scale factor used for addressing elements of the supplied
       * array class.  Where a suitable scale factor can not be returned (e.g.
       * for primitive types), zero should be returned.  The returned value
       * can be used with 
       * <a href="#arrayBaseOffset"><code>arrayBaseOffset</code></a>
       * to access elements of the class.
       * 获取用户给定数组寻址的换算因子.一个合适的换算因子不能返回的时候(例如：基本类型),
       * 返回0.这个返回值能够与<a href="#arrayBaseOffset"><code>arrayBaseOffset</code>
       * </a>一起使用去存取这个数组class中的元素
       * 
       * @param arrayClass the class whose scale factor should be returned.
       * @return the scale factor, or zero if not supported for this array class.
       */
      public native int arrayIndexScale(Class arrayClass);
      
      /***
       * Releases the block on a thread created by 
       * <a href="#park"><code>park</code></a>.  This method can also be used
       * to terminate a blockage caused by a prior call to <code>park</code>.
       * This operation is unsafe, as the thread must be guaranteed to be
       * live.  This is true of Java, but not native code.
       * 释放被<a href="#park"><code>park</code></a>创建的在一个线程上的阻塞.这个
       * 方法也可以被使用来终止一个先前调用<code>park</code>导致的阻塞.
       * 这个操作操作时不安全的,因此线程必须保证是活的.这是java代码不是native代码。
       * @param thread the thread to unblock.
       *           要解除阻塞的线程
       */
      public native void unpark(Thread thread);
      /***
       * Blocks the thread until a matching 
       * <a href="#unpark"><code>unpark</code></a> occurs, the thread is
       * interrupted or the optional timeout expires.  If an <code>unpark</code>
       * call has already occurred, this also counts.  A timeout value of zero
       * is defined as no timeout.  When <code>isAbsolute</code> is
       * <code>true</code>, the timeout is in milliseconds relative to the
       * epoch.  Otherwise, the value is the number of nanoseconds which must
       * occur before timeout.  This call may also return spuriously (i.e.
       * for no apparent reason).
       * 阻塞一个线程直到<a href="#unpark"><code>unpark</code></a>出现、线程
       * 被中断或者timeout时间到期。如果一个<code>unpark</code>调用已经出现了，
       * 这里只计数。timeout为0表示永不过期.当<code>isAbsolute</code>为true时，
       * timeout是相对于新纪元之后的毫秒。否则这个值就是超时前的纳秒数。这个方法执行时
       * 也可能不合理地返回(没有具体原因)
       * 
       * @param isAbsolute true if the timeout is specified in milliseconds from
       *                   the epoch.
       *                   如果为true timeout的值是一个相对于新纪元之后的毫秒数
       * @param time either the number of nanoseconds to wait, or a time in
       *             milliseconds from the epoch to wait for.
       *             可以是一个要等待的纳秒数，或者是一个相对于新纪元之后的毫秒数直到
       *             到达这个时间点
       */
      public native void park(boolean isAbsolute, long time);
    }