CODEWITHSUNDEEP
javamultithreadingsynchronized
NingLee
NingLee

Reputation: 1489

what does synchronized on formal parameter do?

I'm reading the netty source code, and come across a synchronized on formal parameter.

AbstractBootstrap(AbstractBootstrap<B, C> bootstrap) {
    localAddress = bootstrap.localAddress;
    synchronized (bootstrap.options) {
        options.putAll(bootstrap.options);
    }
}

localAddress is not synchronized because it's declared as volatile, any change to it is visable to other thread.

But I don't understand to synchronized on a formal parameter bootstrap.

bootstrap is a formal parameter, every thread has it's own copy. synchronized on it only effect it's own thread? Is this opinion correct?

synchronized (bootstrap.options) is to prevent bootstrap.options to be modified outsides this class or to prevent this.options to be modified by other thread?

Upvotes: 2

Views: 79

Answers (3)

Chris K
Chris K

Reputation: 11907

bootstrap is a formal parameter, every thread has it's own copy. synchronized on it only effect it's own thread? Is this opinion correct?

No, this is not strictly correct. The parameter bootstrap contains a reference to an object, it is not a local copy of the object. Therefore other threads could have their own references to the same instance of the object.

synchronized (bootstrap.options) is to prevent bootstrap.options to be modified outsides this class or to prevent this.options to be modified by other thread?

Kinda. It will block until the current thread can acquire exclusive access to the the object that is in bootstrap.options. It appears to be doing this because bootstrap.options is some kind of collection that is not itself thread safe, and so it is protecting the behaviour of the map under concurrent usage.

Upvotes: 2

Mohit
Mohit

Reputation: 1755

Most likely, this looks to be the case to lock the bootstrap options (some mutable map) while merging options. As there is no additional source code, what I can infer is that bootstrap instance (which is passed as parameter) and its options field is shared between different threads. That's why the synchronization is required.

Upvotes: 0

Esteban Aliverti
Esteban Aliverti

Reputation: 6322

Every constructor invocation receives a copy of the reference variable pointing to an AbstractBootstrap instance.

If you reference another instance within the constructor, the change will only affect the copy that constructor received. But any change in the AbstractBootstrap instance will be reflected to all the threads sharing it.

Hope it helps,

Upvotes: 0

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