StackOverFlowError is one of the common confronted JVM error. In this blog post, lets learn inner mechanics of thread stacks, reasons that can trigger StackOverFlowError and potential solutions to address this error.

To gain deeper understanding into StackOverFlowError, let’s review this simple program:

public class SimpleExample {
 
      public static void main(String args[]) {
 
            a()
 
      }
 
      public static void a() {
 
            int x = 0;
 
            b();
 
      }
 
      public static void b() {
 
            Car y = new Car();
 
            c();
 
      }
 
      public static void c() {
 
            float z = 0f;
 
      System.out.println("Hello");
 
      }
 
}

This program is very simple with the following execution code:

  1. main() method is invoked first
  2. main() method invokes a() method. Inside a() method integer variable ‘x’ is initialized to value 0.
  3. a() method in turn invokes b() method. Inside b() method Car object is constructed and assigned to variable ‘y’.
  4. b() method in turn invokes c() method. Inside c() method float variable ‘z’ is initialized to value 0.



Now let’s review what happens behind the scenes when above simple program is executed. Each thread in the application has its own stack. Each stack has multiple stack frames. Thread adds the methods it’s executing, primitive data types, object pointers, return values to its stack frame in the sequence order in which they are executed.

thread-stack-frame-1.jpg
Fig 1: Thread’s Stack frame

In step #1: main() method is pushed into the application thread’s stack.

In step #2: a() method is pushed into application thread’s stack. In a() method, primitive data type ‘int’ is defined with value 0 and assigned to variable x. This information is also pushed into the same stack frame. Note both data i.e. ‘0’ and variable ‘x’ is pushed into thread’s stack frame.

In step #3: b() method is pushed into thread’s stack. In b() method, ‘Car’ object is created and assigned to variable ‘y’. Crucial point to note here is ‘Car’ object is created in the heap and not in the thread’s stack. Only Car object’s reference i.e. y is stored in the thread’s stack frame.

In step #4: c() method is pushed into thread’s stack. In c() method, primitive data type ‘float’ is defined with value 0f and assigned to variable z. This information is also pushed into same stack frame. Note both data i.e. ‘0f’ and variable ‘z’ is pushed into thread’s stack frame.

Once each method’s execution is completed, then method and the variables/object pointers which are stored in the stack frame are removed as shown in Fig 2.

thread-stack-frame-2.jpg
Fig 2: Thread’s stack frame after executing methods

What causes StackOverflowError?
As you can see thread’s stack is storing methods it’s executing, primitive datatypes, variables, object pointers, and return values. All of these consume memory. If thread’s stack sizes grow beyond the allocated memory limit then StackOverflowError is thrown. Let’s look at the below buggy program, which will result in StackOverflowError:

public class SOFDemo {
 
         public static void a() {
 
                  // Buggy line. It will cause method a() to be called infinite number of times.
 
                  a();
 
         }
 
         public static void main(String args[]) {
 
                   a();
 
         }
 
}

In this program main() method invokes a() method. a() method recursively calls itself. This implementation will cause a() method to be invoked infinite number of times. In this circumstance a() method will be added to thread’s stack frame infinite number of times. Thus, after a few thousand iterations thread’s stack size limit would be exceeded. Once stack size limit is exceeded it will result in ‘StackOverflowError’:

Exception in thread "main" java.lang.StackOverflowError
 
       at com.buggyapp.stackoverflow.SOFDemo.a(SOFDemo.java:7)
 
       at com.buggyapp.stackoverflow.SOFDemo.a(SOFDemo.java:7)
 
       at com.buggyapp.stackoverflow.SOFDemo.a(SOFDemo.java:7)
 
       at com.buggyapp.stackoverflow.SOFDemo.a(SOFDemo.java:7)
 
       at com.buggyapp.stackoverflow.SOFDemo.a(SOFDemo.java:7)
 
       at com.buggyapp.stackoverflow.SOFDemo.a(SOFDemo.java:7)
 
       at com.buggyapp.stackoverflow.SOFDemo.a(SOFDemo.java:7)
 
       at com.buggyapp.stackoverflow.SOFDemo.a(SOFDemo.java:7)

stackoverflowerror.jpg
Fig 3: StackOverflowError progression

What are the solutions to StackOverflowError?
There are couple of strategies to address StackOverflowError.

1. Fix the code
Because of a non-terminating recursive call (as shown in the above example), threads stack size can grow to a large size. In those circumstance, you must fix the source code which is causing recursive looping. When ‘StackOverflowError’ is thrown, it will print the stacktrace of the code that it was recursively executing. This code is a good pointer to start debugging and fixing the issue. In the above example it’s ‘a()’ method.

2. Increase Thread Stack Size (-Xss)
There might be legitimate reason where a threads stack size needs to be increased. Maybe thread has to execute large number of methods or lot of local variables/created in the methods thread has been executing. In such circumstance, you can increase the thread’s stack size using the JVM argument: ‘-Xss’. This argument needs to be passed when you start the application. Example:

-Xss2m

This will set the thread’s stack size to 2 mb.

It might bring a question, what is the default thread’s stack size? Default thread stack size varies based on your operating system, java version & vendor.

JVM version Thread stack size
Sparc 32-bit JVM 512k
Sparc 64-bit JVM 1024k
x86 Solaris/Linux 32-bit JVM 320K
x86 Solaris/Linux 64-bit JVM 1024K
Windows 32-bit JVM 320K
Windows 64-bit JVM 1024K