当他们说StringBuffer类已同步时，这到底是什么意思？ 是否已声明该类的所有方法同步？

Question

I know that StringBuffer class is synchronized(or thread-safe) while StringBuilder is not. Does this mean that all the methods within StringBuffer such as append are declared with the keyword synchronized ?

I checked the hava API: http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuffer.html

As per the API, methods such as public StringBuffer append(String str) are not declared with the keyword synchronized.

Can anyone explain?

Answer 1

The synchronized keyword doesn't show up in the generated JavaDoc, but if you open the source code of StringBuffer , you'll see that each public method which can change the state of the instance actually has synchronized keyword in its signature, eg.

public synchronized StringBuffer append(StringBuffer sb) {
    toStringCache = null;
    super.append(sb);
    return this;
}

As others mentioned in the comments static doesn't directly relate to thread safety. If you meant that the methods are not declared with synchronized keyword, then this should answer your question.

Answer 2

"Thread safety" is a slippery idea.

When somebody says that a class such as StringBuffer is "thread safe" they usually mean that an instance of the class will not behave in weird, unexpected ways when it is accessed by more than one thread. For example, if thread A tries to append "yellow" to a StringBuffer at the same time as thread B appends "green", the final value might be "yellowgreen" or it might be "greenyellow", but it won't be "yegrelleown". No IllegalStateException will be thrown, and it won't crash the JVM.

Those guarantees make each individual instance of the class thread safe, but they won't make your program thread safe. If your program has some thread C that tries to read the StringBuffer at the same time, then it might get either of the two values above, or it might get just "yellow", or just "green", or even the empty string. If it's important for thread C to see both color names in the string, then you will need additional synchronization to guarantee that it happens that way.

Building a program entirely out of "thread-safe" classes does not automatically make the whole program "thread safe."

As other answers have already told you, the StringBuffer class achieves thread-safety by making all of its methods synchronized , but that is not the only way to achieve thread-safety, and in some cases, that may not be enough to achieve it. If you want to know what is thread-safe and what isn't, it's important to read the documentation.

Also, when you read it, pay attention to how the class was meant to be used. If somebody advertizes a class as "thread safe" in some particular use-case, that doesn't mean it will be "thread safe" in other use cases