JVM如何在内部处理竞争条件？

Question

If multiple threads try to update the same member variable, it is called a race condition. But I was more interested in knowing how the JVM handles it internally if we don't handle it in our code by making it synchronised or something else? Will it hang my program? How will the JVM react to it? I thought the JVM would temporarily create a sync block for this situation, but I'm not sure what exactly would be happening.

If any of you have some insight, it would be good to know.

Answer 1

The precise term is a data race , which is a specialization of the general concept of a race condition . The term data race is an official, precisely specified concept, which means that it arises from a formal analysis of the code.

The only way to get the real picture is to go and study the Memory Model chapter of the Java Language Specification, but this is a simplified view: whenever you have a data race, there is almost no guarantee as to the outcome and a reading thread may see any value which has ever been written to the variable. Therein also lies the only guarantee: the thread will not observe an "out-of-thin-air" value, such which was never written. Well, unless you're dealing with long s or double s, then you may see torn writes.

Answer 2

Maybe I'm missing something but what is there to handle? There is still a thread that will get there first. Depending on which thread that is, that thread will just update/read some variable and proceed to the next instruction. It can't magically construct a sync block, it doesn't really know what you want to do. So in other words what happens will depend on the outcome of the 'race'.

Note I'm not heavily into the lower level stuff so perhaps I don't fully understand the depth of your question.

Answer 3

Java provides synchronized and volatile to deal with these situations. Using them properly can be frustratingly difficult, but keep in mind that Java is only exposing the complexity of modern CPU and memory architectures. The alternatives would be to always err on the side of caution, effectively synchronizing everything which would kill performance; or ignore the problem and offer no thread safety whatsoever. And fortunately, Java provides excellent high-level constructs in the java.util.concurrent package, so you can often avoid dealing with the low-level stuff.

Answer 4

In short, the JVM assumes that code is free of data races when translating it into machine code. That is, if code is not correctly synchronized, the Java Language Specification provides only limited guarantees about the behavior of that code.

Most modern hardware likewise assumes that code is free of data races when executing it. That is, if code is not correctly synchronized, the hardware makes only limited guarantees about the result of its execution.

In particular, the Java Language Specification guarantees the following only in the absence of a data race:

• visibility : reading a field yields the value last assigned to it (it is unclear which write was last , and writes of long or double variables need not be atomic )

• ordering : if a write is visible, so are any writes preceding it. For instance, if one thread executes:

   x = new FancyObject(); 

  another thread can read x only after the constructor of FancyObject has executed completely.

In the presence of a data race, these guarantees are null and void. It is possible for a reading thread to never see a write. It is also possible to see the write of x , without seeing the effect of the constructor that logically preceded the write of x . It is very unlikely that the program is correct if such basic assumptions can not be made.

A data race will however not compromise the integrity of the Java Virtual Machine. In particular, the JVM will not crash or halt, and still guarantee memory safety (ie prevent memory corruption) and certain semantics of final fields .

Answer 5

The JVM will handle the situation just fine (ie it will not hang or complain), but you may not get a result that you like!

When multiple threads are involved, java becomes fiendishly complicated and even code that looks obviously correct can turn out to be horribly broken. As an example:

public class IntCounter {
    private int i;

    public IntCounter(int i){
         this.i = i;
    }

    public void incrementInt(){
        i++;
    }

    public int getInt(){
        return i;
    }
}

is flawed in many ways.

First, let's say that i is currently 0 and thread A and thread B both call incrementInt() at about the same time. There is a danger that they will both see that i is 0, then both increment it 1 and then save the result. So at the end of the two calls, i is only 1, not 2!

That's the race condition problem with the code, but there are other problems concerning memory visibility. When thread A changes a shared variable, there is no guarantee (without synchronization) that thread B will ever see the changes!

So thread A could increment i 100 times, and an hour later, thread B, calling getInt(), might see i as 0, or 100 or anywhere in between!

The only sane thing to do if you are delving into java concurrency is to read Java Concurrency in Practice by Brian Goetz et al. (OK there's probably other good ways to learn about it, but this is a great book co written by Joshua Bloch, Doug Lea and others)