多个线程访问的方法相同

Question

I am having a method that is accessed by multiple threads in the java program.This method have:

1. Global object variables
2. Local variable
3. Formal Parameters.

Now this method is not synchronized so when the multiple threads changes the value of above three types of variables then will these changes reflected in other threads or each thread will have separate copy of these variables.

Answer 1

Global object : Will reflect changes

Local variable : Will not

Formal Parameters : Object references Will reflect changes and parameter of primitive type will not reflect changes.

Answer 2

Some of the other answers here forget to mention a very important fact.

Changes to objects passed as parameters and global variables are shared with other threads, however:

Without synchronization you will never know when (perhaps never!) these changes will be shown in other threads. Besides allowing only one thread in the method at the same time preventing 100 flavors of race conditions, entering and exiting a synchronized block will also trigger all caches to be cleared and data to be shared amongst threads/cores.

Multithreading is extremely hard to get right, read up on it carefully. I recommend 'Java Concurrency in Practice' by Goetz.

Answer 3

Golbal objects will be changed, Local vars will not Parameters can be changed if they are objects, if they are primitive types like integer they will not be. Even if they are objects they might not change if every thread creates them just before calling the method and they are not reused.

But that's not all, since you cannot control the order of execution they will change randomly even when one thread is processing so if you have say a global variable int myVar =0;

and the method does

{  // line 10
        myVar = 1;  // line 11
        myVar += 7;  // line 12
    }  // line 13

there is no gurantee at all that the variable will be 1 at the beginning of line 12 since some other thread might have changed the value in the time it takes the program to go from line 11 to line 12.

Bottom line this is a recipe for dissaster. You need to either synchronize the method or add a lock to the global variables and the objects you pass as parameters.