Class DesignForExtensionCheck

  • All Implemented Interfaces:
    Configurable, Contextualizable

    public class DesignForExtensionCheck
    extends AbstractCheck

    Checks that classes are designed for extension (subclass creation).

    Nothing wrong could be with founded classes. This check makes sense only for library projects (not application projects) which care of ideal OOP-design to make sure that class works in all cases even misusage. Even in library projects this check most likely will find classes that are designed for extension by somebody. User needs to use suppressions extensively to got a benefit from this check, and keep in suppressions all confirmed/known classes that are deigned for inheritance intentionally to let the check catch only new classes, and bring this to team/user attention.

    ATTENTION: Only user can decide whether a class is designed for extension or not. The check just shows all classes which are possibly designed for extension. If smth inappropriate is found please use suppression.

    ATTENTION: If the method which can be overridden in a subclass has a javadoc comment (a good practice is to explain its self-use of overridable methods) the check will not rise a violation. The violation can also be skipped if the method which can be overridden in a subclass has one or more annotations that are specified in ignoredAnnotations option. Note, that by default @Override annotation is not included in the ignoredAnnotations set as in a subclass the method which has the annotation can also be overridden in its subclass.

    Problem is described at "Effective Java, 2nd Edition by Joshua Bloch" book, chapter "Item 17: Design and document for inheritance or else prohibit it".

    Some quotes from book:

    The class must document its self-use of overridable methods. By convention, a method that invokes overridable methods contains a description of these invocations at the end of its documentation comment. The description begins with the phrase “This implementation.”
    The best solution to this problem is to prohibit subclassing in classes that are not designed and documented to be safely subclassed.
    If a concrete class does not implement a standard interface, then you may inconvenience some programmers by prohibiting inheritance. If you feel that you must allow inheritance from such a class, one reasonable approach is to ensure that the class never invokes any of its overridable methods and to document this fact. In other words, eliminate the class’s self-use of overridable methods entirely. In doing so, you’ll create a class that is reasonably safe to subclass. Overriding a method will never affect the behavior of any other method.

    The check finds classes that have overridable methods (public or protected methods that are non-static, not-final, non-abstract) and have non-empty implementation.

    Rationale: This library design style protects superclasses against being broken by subclasses. The downside is that subclasses are limited in their flexibility, in particular they cannot prevent execution of code in the superclass, but that also means that subclasses cannot corrupt the state of the superclass by forgetting to call the superclass's method.

    More specifically, it enforces a programming style where superclasses provide empty "hooks" that can be implemented by subclasses.

    Example of code that cause violation as it is designed for extension:

     public abstract class Plant {
       private String roots;
       private String trunk;
    
       protected void validate() {
         if (roots == null) throw new IllegalArgumentException("No roots!");
         if (trunk == null) throw new IllegalArgumentException("No trunk!");
       }
    
       public abstract void grow();
     }
    
     public class Tree extends Plant {
       private List leaves;
    
       @Overrides
       protected void validate() {
         super.validate();
         if (leaves == null) throw new IllegalArgumentException("No leaves!");
       }
    
       public void grow() {
         validate();
       }
     }
     

    Example of code without violation:

     public abstract class Plant {
       private String roots;
       private String trunk;
    
       private void validate() {
         if (roots == null) throw new IllegalArgumentException("No roots!");
         if (trunk == null) throw new IllegalArgumentException("No trunk!");
         validateEx();
       }
    
       protected void validateEx() { }
    
       public abstract void grow();
     }
     
    • Property ignoredAnnotations - Specify annotations which allow the check to skip the method from validation. Type is java.lang.String[]. Default value is After, AfterClass, Before, BeforeClass, Test.
    • Property requiredJavadocPhrase - Specify the comment text pattern which qualifies a method as designed for extension. Supports multi-line regex. Type is java.util.regex.Pattern. Default value is ".*".

    To configure the check:

     <module name="DesignForExtension"/>
     

    To configure the check to allow methods which have @Override and @Test annotations to be designed for extension.

     <module name="DesignForExtension">
       <property name="ignoredAnnotations" value="Override, Test"/>
     </module>
     
     public class A {
       @Override
       public int foo() {
         return 2;
       }
    
       public int foo2() {return 8;} // violation
     }
    
     public class B {
       /**
        * This implementation ...
          @return some int value.
        */
       public int foo() {
         return 1;
       }
    
       public int foo3() {return 3;} // violation
     }
    
     public class FooTest {
       @Test
       public void testFoo() {
         final B b = new A();
         assertEquals(2, b.foo());
       }
    
       public int foo4() {return 4;} // violation
     }
     

    To configure the check to allow methods which contain a specified comment text pattern in their javadoc to be designed for extension.

     <module name="DesignForExtension">
       <property name="requiredJavadocPhrase"
         value="This implementation"/>
     </module>
     
     public class A {
       /**
       * This implementation ...
       */
       public int foo() {return 2;} // ok, required javadoc phrase in comment
    
       /**
       * Do not extend ...
       */
       public int foo2() {return 8;} // violation, required javadoc phrase not in comment
    
       public int foo3() {return 3;} // violation, required javadoc phrase not in comment
     }
     

    To configure the check to allow methods which contain a specified comment text pattern in their javadoc which can span multiple lines to be designed for extension.

     <module name="DesignForExtension">
       <property name="requiredJavadocPhrase"
         value="This[\s\S]*implementation"/>
     </module>
     
     public class A {
       /**
       * This
       * implementation ...
       */
       public int foo() {return 2;} // ok, required javadoc phrase in comment
    
       /**
       * Do not extend ...
       */
       public int foo2() {return 8;} // violation, required javadoc phrase not in comment
    
       public int foo3() {return 3;} // violation, required javadoc phrase not in comment
     }
     

    Parent is com.puppycrawl.tools.checkstyle.TreeWalker

    Violation Message Keys:

    • design.forExtension
    Since:
    3.1
    • Field Detail

      • ignoredAnnotations

        private Set<String> ignoredAnnotations
        Specify annotations which allow the check to skip the method from validation.
      • requiredJavadocPhrase

        private Pattern requiredJavadocPhrase
        Specify the comment text pattern which qualifies a method as designed for extension. Supports multi-line regex.
    • Method Detail

      • setIgnoredAnnotations

        public void setIgnoredAnnotations​(String... ignoredAnnotations)
        Setter to specify annotations which allow the check to skip the method from validation.
        Parameters:
        ignoredAnnotations - method annotations.
      • setRequiredJavadocPhrase

        public void setRequiredJavadocPhrase​(Pattern requiredJavadocPhrase)
        Setter to specify the comment text pattern which qualifies a method as designed for extension. Supports multi-line regex.
        Parameters:
        requiredJavadocPhrase - method annotations.
      • getAcceptableTokens

        public int[] getAcceptableTokens()
        Description copied from class: AbstractCheck
        The configurable token set. Used to protect Checks against malicious users who specify an unacceptable token set in the configuration file. The default implementation returns the check's default tokens.
        Specified by:
        getAcceptableTokens in class AbstractCheck
        Returns:
        the token set this check is designed for.
        See Also:
        TokenTypes
      • hasJavadocComment

        private boolean hasJavadocComment​(DetailAST methodDef)
        Checks whether a method has a javadoc comment.
        Parameters:
        methodDef - method definition token.
        Returns:
        true if a method has a javadoc comment.
      • hasJavadocCommentOnToken

        private boolean hasJavadocCommentOnToken​(DetailAST methodDef,
                                                 int tokenType)
        Checks whether a token has a javadoc comment.
        Parameters:
        methodDef - method definition token.
        tokenType - token type.
        Returns:
        true if a token has a javadoc comment.
      • branchContainsJavadocComment

        private boolean branchContainsJavadocComment​(DetailAST token)
        Checks whether a javadoc comment exists under the token.
        Parameters:
        token - tree token.
        Returns:
        true if a javadoc comment exists under the token.
      • hasValidJavadocComment

        private boolean hasValidJavadocComment​(DetailAST detailAST)
        Checks whether a javadoc contains the specified comment pattern that denotes a method as designed for extension.
        Parameters:
        detailAST - the ast we are checking for possible extension
        Returns:
        true if the javadoc of this ast contains the required comment pattern
      • isNativeMethod

        private static boolean isNativeMethod​(DetailAST ast)
        Checks whether a methods is native.
        Parameters:
        ast - method definition token.
        Returns:
        true if a methods is native.
      • hasEmptyImplementation

        private static boolean hasEmptyImplementation​(DetailAST ast)
        Checks whether a method has only comments in the body (has an empty implementation). Method is OK if its implementation is empty.
        Parameters:
        ast - method definition token.
        Returns:
        true if a method has only comments in the body.
      • canBeOverridden

        private static boolean canBeOverridden​(DetailAST methodDef)
        Checks whether a method can be overridden. Method can be overridden if it is not private, abstract, final or static. Note that the check has nothing to do for interfaces.
        Parameters:
        methodDef - method definition token.
        Returns:
        true if a method can be overridden in a subclass.
      • hasIgnoredAnnotation

        private static boolean hasIgnoredAnnotation​(DetailAST methodDef,
                                                    Set<String> annotations)
        Checks whether a method has any of ignored annotations.
        Parameters:
        methodDef - method definition token.
        annotations - a set of ignored annotations.
        Returns:
        true if a method has any of ignored annotations.
      • getAnnotationName

        private static String getAnnotationName​(DetailAST annotation)
        Gets the name of the annotation.
        Parameters:
        annotation - to get name of.
        Returns:
        the name of the annotation.
      • getNearestClassOrEnumDefinition

        private static DetailAST getNearestClassOrEnumDefinition​(DetailAST ast)
        Returns CLASS_DEF or ENUM_DEF token which is the nearest to the given ast node. Searches the tree towards the root until it finds a CLASS_DEF or ENUM_DEF node.
        Parameters:
        ast - the start node for searching.
        Returns:
        the CLASS_DEF or ENUM_DEF token.
      • canBeSubclassed

        private static boolean canBeSubclassed​(DetailAST classDef)
        Checks if the given class (given CLASS_DEF node) can be subclassed.
        Parameters:
        classDef - class definition token.
        Returns:
        true if the containing class can be subclassed.
      • hasDefaultOrExplicitNonPrivateCtor

        private static boolean hasDefaultOrExplicitNonPrivateCtor​(DetailAST classDef)
        Checks whether a class has default or explicit non-private constructor.
        Parameters:
        classDef - class ast token.
        Returns:
        true if a class has default or explicit non-private constructor.