Why java recursively calling function can run forever even though StackOverflowError?

user_10969942 0
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I met the following problem in a Java exam, why following recursively calling a function can run forever even though StackOverflowError?

public class Solution {
    static int i = 0;

    public static void f(){
        System.out.println(i++);
        try {
            f();
        } catch (StackOverflowError e) {
            System.out.println(e);
            f();
        }
    }

    public static void main(String[] args) {
        f();
    }
}

I cannot understand why JVM can still run when I've exhausted all call stack memory? Is there any reference, like JVM specification or JVM source code, can explain above phenomenon?

jvm java
1 Answers

I'm neither java nor jvm expert, but I think adding a extra recursive depth output can show exactly what is going on with the call stack.

public class Solution {
  static int i = 0;

  public static void f(int depth) {
    System.out.println(i++);
    System.out.println(String.format("depth=%d", ++depth));
    try {
      f(depth);
    } catch (StackOverflowError e) {
      System.out.println(e);
      f(depth);
    }
  }

  public static void main(String[] args) {
    f(1);
  }
}

In my test, typical output is

...more
java.lang.StackOverflowError
161735
depth=3745
161736
depth=3746
161737
java.lang.StackOverflowError
161738
java.lang.StackOverflowError
161739
java.lang.StackOverflowError
161740
depth=3744
161741
depth=3745
...more

StackOverflowError is thrown because jvm is considering a size limit(1M by default?) of stack memory has reached, but the exception is caught, so the recursion is not stopped, it still tries to call, jvm throw away the recent 1 or 2 problematic calls, and then try continue, so the depth value goes around the same maximum value again and again.

I think this way the OP can understand why i is increasing without any problem. I've exhaust the call stack memory -- you haven't, you nearly exhaust, but jvm always cleared latest few calls to avoid exhaust.

user_15181000
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