Thread synchronization and the synchronized modifier

[A GJP]

Hello,

I am currently developing a program that makes use of three Vectors to store connection information. These Vectors are accessed by multiple Threads that modify their data and most importantly, use methods that remove information from one Vector, converts it and then puts it into a different one.

I do know that Vectors are synchronized, but since the methods contain multiple (synchronized) modifications in a row, I decided to synchronize these methods too in order to prevent everything getting mixed up.

What I'm wondering about is:

A) Is it really necessary to synchronize the entire method?
B) Does the synchronized method modifier do anything else except creating a synchronized block with a monitor on this around the content?
C) What is the point of allowing synchronized static methods then?

God, lots of synchronizeds in this post O_o

Edit: Urgh, sorry. I just noticed that there is a section for synchronization. Would be nice if someone could move it there.

[eRaaaa]

a) maybe? if you want or if it`s required that the method is executed sequentially one after the other (atomic), then yes.
b) not really
c) you will lock the class and not an object!

Intrinsic Locks and Synchronization (The Java™ Tutorials > Essential Classes > Concurrency)

[j2me64]

in the scjp book (p. 744) you can read

the moral here is that just because a class is described as "thread-safe" doesn't mean it is always thread-safe. The individual methods are synchronized, that may not be enough - you may be better off putting in synchronization at a higher level (i.e., put it in the block or method that calls the other methods).

[kjkrum]

A) Not necessarily. You could also use synchronized blocks:

synchronized(myVector) {
    // several modifications
}

If you do this, you could use a Collections class instead of a Vector, which is recommended. Vectors are deprecated.

C) synchronized static methods synchronize on the Class object representing the class itself, rather than on an instance object.

[doWhile]

Vectors are deprecated.

Just a clarification: Vector is not currently deprecated. But to quote the API from java 7:

If a thread-safe implementation is not needed, it is recommended to use ArrayList in place of Vector.

The class does contain legacy code, as it was created prior to the Collections framework and adapter later to conform to it.
List Implementations (The Java™ Tutorials > Collections > Implementations)