Thread Safety: Maximum efficiency-CodePudding

Good Evening,

I am trying to understand how I am using multi-threading and how to implement thread-safety in the context.

When I want to achieve maximum speed of my threads do I use:

    public void addMarketOrder(MarketOrder marketOrder) {
    if (marketOrder.id != this.id) {
        return;
    }
    synchronized (this) {
        ordered  = marketOrder.ordered;
    }
}

or just synchronized the entire method?

    public synchronized void addMarketOrder(MarketOrder marketOrder) {
    if (marketOrder.id != this.id) {
        return;
    }
    ordered  = marketOrder.ordered;
}

CodePudding user response：

Assuming the ids do not change, the first case is preferable to the second. The first case avoids synchronization if the ids do not match. The second case synchronizes even if there isn't any write operation.

CodePudding user response：

What you are hinting towards is double check locking. The correct form is:

    public void addMarketOrder(MarketOrder marketOrder) {
    if (marketOrder.id != this.id) {
        return;
    }
    synchronized (this) {
        if (marketOrder.id != this.id) {
           ordered  = marketOrder.ordered;
        }
    }
}

Because you shouldn't assume that because the condition became true that it will continue to be true.

Also if you read the id without synchronization it should be volatile to avoid the value getting masked by the cache in multiple core machines.

You can also synchronize the whole method without any problem. But synchronizing the whole method will make it less efficient (which could matter if you have a lot of processors and this is a highly contested method).