原文: 第五章







What was wrong with the old memory model?

There were several serious problems with the old memory model. It was difficult to understand, and therefore widely violated. For example, the old model did not, in many cases, allow the kinds of reorderings that took place in every JVM. This confusion about the implications of the old model was what compelled the formation of JSR-133.

One widely held belief, for example, was that if final fields were used, then synchronization between threads was unnecessary to guarantee another thread would see the value of the field. While this is a reasonable assumption and a sensible behavior, and indeed how we would want things to work, under the old memory model, it was simply not true. Nothing in the old memory model treated final fields differently from any other field — meaning synchronization was the only way to ensure that all threads see the value of a final field that was written by the constructor. As a result, it was possible for a thread to see the default value of the field, and then at some later time see its constructed value. This means, for example, that immutable objects like String can appear to change their value — a disturbing prospect indeed.

The old memory model allowed for volatile writes to be reordered with nonvolatile reads and writes, which was not consistent with most developers intuitions about volatile and therefore caused confusion.

Finally, as we shall see, programmers’ intuitions about what can occur when their programs are incorrectly synchronized are often mistaken. One of the goals of JSR-133 is to call attention to this fact.

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