ERR53-J. Try to gracefully recover from system errors

According to the Java Language Specification, §11.1.1, "The Kinds of Exceptions" [JLS 2013],

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 are not subject to compile-time checking because it is tedious to account for all exceptional conditions and because recovery is often difficult or impossible. 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 because it makes handling specific exceptions difficult. Where cleanup operations such as releasing system resources can be performed, code should use a finally block to release the resources or a try-with-resources statement.

Catching Throwable is disallowed in general by ERR08-J. Do not catch NullPointerException or any of its ancestors, but it is permitted when filtering exception traces 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 constrained memory conditions because the stack or heap may be nearly exhausted. In such a scenario, one useful technique is 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 abruptly terminate a Java program may result in a denial-of-service vulnerability.

Bibliography