I am trying to use synchronized block for my use-case where I need to use it on following code.

public void saveInCatch(String reqId,String desc) {
    synchronized (reqId) {
        cache.add(reqId,desc);
    }
}

This method is getting called from many places in my application. I want to make sure that the code block should be accessed by only one thread at a time if the reqId is same. If it is different then this block can be accessed concurrently. How can I achieve this?

CodePudding user response：

You are looking when one thread is wiritting still another thread should be able to read it

I think that would be tricky to do. Since the value should be flushed out to memory.

Check the link for similar question

Java Threads: synchronized reading and writing of value on the same object

CodePudding user response：

The synchronized block works on the instance Id of the object you are matching

so with your code I can create two strings

String id1 = "1234";
String id2 = "1234";

If two threads call your function using different variables containing the same Id then they will not get blocked, they will both run.

If you really want to sychronize on a string like that you should create static final String objects for every possible Id

Note: if you decide to use a numeric type like Integer for you sync object then you might find it works for low numbers and then stops working as numbers get higher as Java uses a table of Ints so that only one version of 1 will be created for your code, but every instance of 9999 might have a unique object Id