Unsafe 类的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 (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); }