java不能直接访问操作系统底层，而是通过本地方法来访问。Unsafe类提供了硬件级别的原子操作(直接引用网上的这句话,大致意思就是我们只能使用应用层的东西,然后应用层会去修改内存中对应的东西,也就是上面说的底层,但是我们一般是不能直接修改内存中东西的,如果人为的破坏了系统怎么办,这事一个安全问题)
线程的挂起与恢复
将一个线程进行挂起是通过park方法实现的，调用 park后，线程将一直阻塞直到超时或者中断等条件出现。unpark可以终止一个挂起的线程，使其恢复正常。整个并发框架中对线程的挂起操作被封装在 LockSupport类中，LockSupport类中有各种版本pack方法，但最终都调用了Unsafe.park()方法。
public class LockSupport {
public static void unpark(Thread thread) {
if (thread != null)
unsafe.unpark(thread);
}
public static void park(Object blocker) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(false, 0L);
setBlocker(t, null);
}
public static void parkNanos(Object blocker, long nanos) {
if (nanos > 0) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(false, nanos);
setBlocker(t, null);
}
}
public static void parkUntil(Object blocker, long deadline) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(true, deadline);
setBlocker(t, null);
}
public static void park() {
unsafe.park(false, 0L);
}
public static void parkNanos(long nanos) {
if (nanos > 0)
unsafe.park(false, nanos);
}
public static void parkUntil(long deadline) {
unsafe.park(true, deadline);
}
}
CAS操作
/**
* 比较obj的offset处内存位置中的值和期望的值，如果相同则更新。此更新是不可中断的。
*
* @param obj 需要更新的对象
* @param offset obj中整型field的偏移量
* @param expect 希望field中存在的值
* @param update 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 如果field的值被更改返回true
*/
public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);
这个方法相对来说看起来会熟悉一点
AbstractQueuedSynchronizer#compareAndSetTail
private final boolean compareAndSetTail(Node expect, Node update) {
return U.compareAndSwapObject(this, TAIL, expect, update);
}
上面的方法是AbstractQueuedSynchronizer中修改队列队尾的CAS操作,参数如下：
Node expect 需要修改的对象中的某一个属性的预期值(也就是猜测这个属性现在的内存值是不是这个
expect值)
Node update 需要修改的对象中的某一个属性需要被修改的指定值
TAIL 需要修改的对象中的某一属性对象
this 需要被修改的对象
我们先看下TAIL是怎么来的
在AbstractQueuedSynchronizer的静态代码块中有这样的一段代码
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
private static final long STATE;
private static final long HEAD;
private static final long TAIL;
static {
try {
STATE = U.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
HEAD = U.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
TAIL = U.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
} catch (ReflectiveOperationException e) {
throw new Error(e);
}
// Reduce the risk of rare disastrous classloading in first call to
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
Class ensureLoaded = LockSupport.class;
}
首先初始化Unsafe对象,然后定义了几个静态常量,这些静态常量的赋值是在静态代码块中,通过反射得到属性对应的属性对象,并传给U.objectFieldOffset并返回一个属性值,赋值给相应的属性,TAIL就是其中一个属性
所以上面AbstractQueuedSynchronizer修改队尾的操作完整的意义是:通过比较当前AbstractQueuedSynchronizer对象中的TAIL属性所对应的值是不是expect,如果是则将更新值updata作为一个新值赋值给TAIL属性,并返回true;如果TAIL对象的内存值不是expect,则不进行任何的赋值操作,最后返回false。
下面是从网上找到的一份Unsafe源码 地址 感谢作者
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 ([email protected])
* @author Andrew John Hughes ([email protected])
*/
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 Unsafe. 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Unsafe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 obj.
* obj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 expect.
* 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 obj.
* obj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 expect.
* 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 obj.
* obj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 expect.
* 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 putIntVolatile(Object,long,int), which
* doesn't guarantee the immediate visibility of the change to other
* threads. It is only really useful where the integer field is
* volatile, 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putIntVolatile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volatile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 obj.
* obj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 putLongVolatile(Object,long,long), which
* doesn't guarantee the immediate visibility of the change to other
* threads. It is only really useful where the long field is
* volatile, 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putLongVolatile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volatile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 obj.
* obj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 putObjectVolatile(Object,long,Object), which
* doesn't guarantee the immediate visibility of the change to other
* threads. It is only really useful where the object field is
* volatile, 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putObjectVolatile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volatile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 obj.
* obj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 obj.
* obj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 obj.
* obj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 obj.
* obj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 obj.
* obj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 obj.
* obj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 obj.
* obj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 obj.
* obj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 obj.
* obj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 obj.
* obj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
* arrayIndexScale,
* 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arrayIndexScale
* 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
* arrayBaseOffset
* 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arrayBaseOffset
* 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
* park. This method can also be used
* to terminate a blockage caused by a prior call to park.
* 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park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park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
* unpark occurs, the thread is
* interrupted or the optional timeout expires. If an unpark
* call has already occurred, this also counts. A timeout value of zero
* is defined as no timeout. When isAbsolute is
* true, 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unpark出现、线程
* 被中断或者timeout时间到期。如果一个unpark调用已经出现了，
* 这里只计数。timeout为0表示永不过期.当isAbsolute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);
}
功能大概包括比较修改属性值、直接修改属性值、获取属性值，还有上面说的线程的挂起