Java 中的同步帮助
查看 http://download.eclipse.org/ jetty/stable-7/xref/com/acme/ChatServlet.html，我似乎不明白为什么在同步方法中需要有同步块，如下所示：
private synchronized void chat(HttpServletRequest request,HttpServletResponse response,String username,String message)
throws IOException
{
Map<String,Member> room=_rooms.get(request.getPathInfo());
if (room!=null)
{
// Post chat to all members
for (Member m:room.values())
{
synchronized (m)
{
m._queue.add(username); // from
m._queue.add(message); // chat
// wakeup member if polling
if (m._continuation!=null)
{
m._continuation.resume();
m._continuation=null;
}
}
}
}
Why does m< /code> 如果整个方法已经是线程安全的，是否需要同步（再次？）？
感谢您的任何见解。
looking at http://download.eclipse.org/jetty/stable-7/xref/com/acme/ChatServlet.html, I don't seem to understand why there needs to be a synchronization block in a synchronized method, like so:
private synchronized void chat(HttpServletRequest request,HttpServletResponse response,String username,String message)
throws IOException
{
Map<String,Member> room=_rooms.get(request.getPathInfo());
if (room!=null)
{
// Post chat to all members
for (Member m:room.values())
{
synchronized (m)
{
m._queue.add(username); // from
m._queue.add(message); // chat
// wakeup member if polling
if (m._continuation!=null)
{
m._continuation.resume();
m._continuation=null;
}
}
}
}
Why does m need to be synchronized (again?) if the whole method is already thread-safe?
Thank you for any insight.
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同步方法“chat(...)”在其实例对象上同步，而synchronized(m) 在“m”对象上同步 - 因此它们在两个不同的对象上同步。基本上，它是确保其他一些 servlet 对象不会同时干扰同一个成员实例。
The synchronized method "chat(...)" synchronizes on it's instance object whereas the synchronized(m) synchronizes on the "m" object - so they are synchronizing on two different objects. Basically it's making sure that some other servlet object isn't messing with the same Member instance at the same time.
当整个方法同步时，将在
this对象上获得锁。但同步块仅获取当前迭代中使用的成员的锁。When whole method is synchronized the lock is obtained on the
thisobject. But the synchronized block obtains lock only on the member currently being used in iteration.同步是在不同的锁上。
方法定义中的
synchronized关键字意味着在this上同步的其他代码不能与该方法并行运行。synchronized(m)作用域意味着在m上同步的其他代码无法与循环并行运行。The synchronization is on different locks.
The
synchronizedkeyword at the method definition means that other code that synchronizes onthiscannot run run in parallel to the method.The
synchronized(m)scope means that other code that synchronizes onmcannot run in parallel to the loop.