ERR10-J. Do not let code throw undeclared checked exceptions

Java requires that each method must address every checked exception that can be thrown during its execution either by handling the exception within a try-catch block or by declaring that the exception can propagate out of the method (via the throws clause). Unfortunately, undeclared checked exceptions can be thrown at runtime. Consequently, callers cannot use the throws clause to determine the complete set of checked exceptions that could propagate from an invoked method.

David: your paragraph about checked vs. unchecked exceptions goes here.

Clients or callers need to know which exceptions the underlying code can throw. Consequently, developers should – and developers of security critical software must — sufficiently document all possible unchecked and undeclared checked exceptions that can be thrown by a method.

Noncompliant Code Example

This noncompliant code example is insecure both because it can throw undeclared checked exceptions and also because it uses the sun.misc.Unsafe class. All sun.* classes are unsupported and undocumented because their use can cause portability and backward compatibility issues.

Classes that are loaded by the bootstrap class loader have the permissions needed to call the static factory method Unsafe.getUnsafe(). Arranging to have an arbitrary class loaded by the bootstrap class loader without modifying the sun.boot.class.path system property can be difficult. However, an alternative way to gain access is to change the accessibility of the field that holds an instance of Unsafe through the use of reflection. This works only when permitted by the current security manager (which would violate guideline ENV04-J. Do not grant ReflectPermission with target suppressAccessChecks.) Given access to Unsafe, a call can throw an undeclared checked exception by calling the Unsafe.throwException() method.

import java.io.IOException;
import java.lang.reflect.Field;
import sun.misc.Unsafe;

public class UnsafeCode {
  public static void main( String[] args ) throws SecurityException, NoSuchFieldException,
  IllegalArgumentException, IllegalAccessException {
    Field f = Unsafe.class.getDeclaredField("theUnsafe");
    field.setAccessible(true);
    Unsafe u =  (Unsafe) field.get(null);
    u.throwException(new IOException("No need to declare this checked exception"));
  }
}

Noncompliant Code Example

Any checked exception thrown by the default constructor of Class.newInstance() is propagated to the caller, even though Class.newInstance declares that it throws only InstantiationException and IllegalAccessException. This noncompliant code example demonstrates one way to use Class.newInstance to throw arbitrary exceptions, whether checked or unchecked.

public class BadNewInstance {
  private static Throwable throwable;

  private BadNewInstance() throws Throwable {
    throw throwable;
  }

  public static synchronized void undeclaredThrow(Throwable throwable) {
    // These two should not be passed
    if (throwable instanceof IllegalAccessException || throwable instanceof InstantiationException) {
        throw new IllegalArgumentException(); // Unchecked, no declaration required
    }
       
    BadNewInstance.throwable = throwable;
    try {
      // next line throws the Throwable argument passed in above,
      // even though the throws clause of class.newInstance fails
      // to declare that this may happen; see JavaDoc
      BadNewInstance.class.newInstance();
    } catch (InstantiationException e) { /* unreachable */ } 
      catch (IllegalAccessException e) { /* unreachable */ } 
    finally { // Avoid memory leak 
      BadNewInstance.throwable = null; 
    }  
  }
}

public class UndeclaredException {
  public static void main(String[] args) {   // No declared checked exceptions
    BadNewInstance.undeclaredThrow(new Exception("Any checked exception"));
  }
}

When the programmer wishes to catch and handle the possible undeclared checked exceptions, the compiler refuses to believe that any can be thrown in the particular context. A simple workaround is to catch Exception and dynamically check whether the caught exception is an instance of the possible checked exception (carefully re-throwing all other exceptions, of course), as shown below. This approach is fragile, because any unanticipated checked exception bypasses the dynamic check.

public static void main(String[] args) {
  try {	
    BadNewInstance.undeclaredThrow(new IOException("Any checked exception"));
  } catch(Exception e) { 
    if (e instanceof IOException) {
      System.out.println("IOException occurred");
    } else if (e instanceof RuntimeException) {
      throw (RuntimeException) e;
    } else {
      //some other unknown checked exception
    }
  } 
} 

Compliant Solution

Use method Constructor.newInstance() rather than Class.newInstance(). The java.lang.reflect.Constructor.newInstance() method wraps any exceptions thrown from within the constructor into a checked exception called InvocationTargetException. Use of the builder interface recommended by Bloch [[Bloch 2008]] is an acceptable alternative.

// Generic type for a builder used to build any object of type T
public interface Builder<T> {
 public T build();
}

A client can pass a builder to a method and request the creation of an object. A bounded wildcard type should be used to constrain the builder's type parameter. In the code snippet that follows, a US Dollar (USD) is built from coins of different denomination.

USD buildCurrency(Builder<? extends denomination> currencyBuilder) { /* ... */ }

For further details on implementing the builder pattern, refer to guideline OBJ05-J. Prevent access to partially initialized objects. In the example described in that guideline, the Currency.Builder class must implement the Builder interface highlighted in this recommendation.

Noncompliant Code Example

An unchecked cast of a generic type with parameterized exception declaration can also result in unexpected checked exceptions. The compiler complains unless the warnings are suppressed.

interface Thr<EXC extends Exception> {
  void fn() throws EXC;
}

public class UndeclaredGen {
  static void undeclaredThrow() throws RuntimeException {
    @SuppressWarnings("unchecked")  // Suppresses warnings  
    Thr<RuntimeException> thr = (Thr<RuntimeException>)(Thr)
      new Thr<IOException>() {
        public void fn() throws IOException {
          throw new IOException();
	}
      };
      thr.fn();
    }

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

Noncompliant Code Example

According to the Java API [[API 2006]], class Thread documentation

[Thread.stop()] may be used to generate exceptions that its target thread is unprepared to handle (including checked exceptions that the thread could not possibly throw, were it not for this method). For example, the following method is behaviorally identical to Java's throw operation, but circumvents the compiler's attempts to guarantee that the calling method has declared all of the checked exceptions that it may throw.

static void sneakyThrow(Throwable t) {
  Thread.currentThread().stop(t);
}

It is also possible to disassemble a class, remove any declared checked exceptions and reassemble the class so that checked exceptions are thrown at runtime when the class is used [[Roubtsov 2003]]. Compiling against a class that declares the checked exception and supplying at runtime a class that lacks the declaration also suffices. Similarly, a compiler other than javac might handle checked exceptions differently. Undeclared checked exceptions can also be produced through crafted use of the sun.corba.Bridge class. All these methods are strongly discouraged.

Compliant Solution

Refrain from employing code (whether legitimate or hostile) that can throw undeclared checked exceptions. When this is unavoidable, explicitly document the behavior. Finally, never use deprecated methods such as Thread.stop() (as required by MET15-J. Do not use deprecated or obsolete classes or methods).

Risk Assessment

Failure to document undeclared checked exceptions can result in checked exceptions that the caller is unprepared to handle, consequently violating the safety property.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

[[Bloch 2008]] Item 2: "Consider a builder when faced with many constructor parameters"
[[Goetz 2004b]]
[[JLS 2005]] Chapter 11: Exceptions
[[MITRE 2009]] CWE ID 703 "Failure to Handle Exceptional Conditions", CWE ID 248 "Uncaught Exception"
[[Roubtsov 2003]]
[[Schwarz 2004]]
[[Venners 2003]] "Scalability of Checked Exceptions"

ERR09-J. Prevent inadvertent calls to System.exit() or forced shutdown      06. Exceptional Behavior (EXC)      EXC11-J. Restore prior object state on method failure