34. ERR53-JG. Try to gracefully recover from system errors

According to the Java Language Specification, §11.1.1, "The Kinds of Exceptions" [JLS 2011],

The unchecked exceptions classes are the class RuntimeException and its subclasses, and the class Error and its subclasses. All other exception classes are checked exception classes.

Unchecked exception classes such as Error and its subclasses are not subject to compile-time checking because it is tedious to account for all exceptional conditions and because recovery is generally difficult. However, even when recovery is impossible, the Java Virtual Machine (JVM) allows a graceful exit and a chance to at least log the error. This is made possible by using a try-catch block that catches Throwable. Also, when code must avoid leaking potentially sensitive information, catching Throwable is permitted. In all other cases, catching Throwable is not recommended. Where cleanup operations such as releasing system resources are possible, code should use a finally block within which the resources are released or should use a try-with-resources statement.

Catching Throwable is forbidden by ERR08-J. Do not catch NullPointerException or any of its ancestors but is permitted when filtering exceptions by the exception ERR08-EX0 in that rule.

Noncompliant Code Example

This noncompliant code example generates a StackOverflowError as a result of infinite recursion. It exhausts the available stack space and may result in denial of service.

public class StackOverflow {
  public static void main(String[] args) {
    infiniteRun();
    // ...
  }
    
  private static void infiniteRun() {
    infiniteRun();
  }
}

Compliant Solution

This compliant solution shows a try-catch block that can be used to capture java.lang.Error or java.lang.Throwable. A log entry can be made at this point, followed by attempts to free key system resources in the finally block.

public class StackOverflow {
  public static void main(String[] args) {
    try {
      infiniteRun();
    } catch (Throwable t) {
      // Forward to handler
    } finally {
      // Free cache, release resources
    }
    // ...
  }
    
  private static void infiniteRun() {
    infiniteRun();    
  }
}

Note that the Forward to handler code must operate correctly in very constrained memory conditions because the stack or heap may be nearly exhausted. In such a scenario, one useful technique would be for the program to initially reserve memory specifically to be used by an out-of-memory exception handler.

Note that this solution catches Throwable in an attempt to handle the error; it falls under exception ERR08-EX2 in ERR08-J. Do not catch NullPointerException or any of its ancestors.

Applicability

Allowing a system error to propagate out of a Java program may result in a denial-of-service attack.

Bibliography