hashmap 入口循环中的多线程 - 执行器服务 (Java)

Question

I have a map (HashMap<String, Map<String,String> mapTest) in which I have a loop that does several operations.

HashMap<String, Map<String,String> mapTest = new HashMap<>();
ArrayList<Object> testFinals = new ArrayList<>();

for (Map.Entry<String, Map<String, String>> entry: mapTest.entrySet()) {

// in here I do a lot of things, like another for loops, if's, etc.

//the final juice to get from here is that in each time the loop is executed I have this:

List<Object> resultExp = methodXYZ(String, String, String);

testFinals.addAll(resultExp);

}

- In here, I have to wait before I proceed, since I need the full testFinals filled to advance.

Now, what I need to do is:

1 - This mapTest can have like 400 rows to iterate from.

I want to schedule like 4 threads, and assign like 100 rows of that FOR cycle to thread 1, the next 100 rows of the mapTest to thread 2, and so on.

Already tryed a few solutions, like this one:

ExecutorService taskExecutor = Executors.newFixedThreadPool(4);

while(...) {

  taskExecutor.execute(new MyTask());

}

taskExecutor.shutdown();

try {

  taskExecutor.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);

} catch (InterruptedException e) {

  ...

}

but I can't adapt this correctly or a similar working solution to what I have now, with that map iteration.

Answer 1

When using concurrent, consider that the threads must obtain, hold and relinquish locks on a variable.

This is done at field level -not content.

On short... The hasmap is locked for access ya specific thread. Not some random entry.