object.wait为什么要和synchronized一块使用

Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object. 
In other words, this method behaves exactly as if it simply performs the call wait(0).
The current thread must own this object‘s monitor. 
The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object‘s monitor to wake up 
either through a call to the notify method or the notifyAll method. 
The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
           synchronized (obj) {
               while (<condition does not hold>)
                   obj.wait();
               ... // Perform action appropriate to condition
           }
       
This method should only be called by a thread that is the owner of this object‘s monitor. 
See the notify method for a description of the ways in which a thread can become the owner of a monitor.