Why does the producer / consumer mode wait () use while instead of if (clear and understandable)

One 、 Producer consumer model

Producer consumer pattern is a common pattern in multithreading , What he mainly realizes is how much resources the producer produces , How many resources can consumers consume , When resources are insufficient, consumers need to wait and inform producers to consume , And the producer after the production resources reach the specified limit , We must stop production and wait for consumers to consume , As long as the resource limit is not reached , Producers can produce at any time .

Two 、 Why? wait() Use while Make judgments instead of if Do judgment

1.wait Characteristics of the method

First , We need to know wait() The lock will be released after execution , It can be understood as resetting the lock , That is, the current thread is still waiting , But you can let the next thread enter the lock .

2. Why use while Make judgments instead of if Do judgment

First , If we use if Do judgment , When the first thread enters, it passes if Execute after judgment wait() Wait and release the lock , Then the next thread may also enter and pass if Execute after judgment wait Method waits and releases the lock , Then we will have multiple threads waiting in the same place , When we wake up the thread , Because all threads are wait() It has passed before if Judge , So all threads can execute directly down , This makes us unable to appear One production, one consumption , So there's a problem .

But when we use while When making judgments , When we wake up all threads , Then the first one out while The thread of the loop will modify while Judgment condition , Other threads want to start from while If you come out of the cycle, you need to judge again , Because the condition has been modified by the first thread , Then naturally, I can't get out , So other threads will continue to execute wait() Waiting for the next wake up .

3. My other way of thinking

Personal understanding may not be correct , Please comment and testify if there is any mistake .

I think if I wake up the thread , Don't use this.notifyAll() Wake up all threads , It's just using this.notify() Wake up a thread randomly , So actually use while and if It can be achieved by making judgments One production, one consumption The request has been made. .