Unsafe API说明

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 ([e-mail]) * @author Andrew John Hughes ([e-mail]) */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);}