<?xml version="1.0" encoding="ISO-8859-1"?>

<document>
  <properties>
    <title>Difference Message Reference Manual</title>
    <author>The Clirr Development Team</author>
  </properties>
  <body>
    <section name="Introduction">
      <p>
        When clirr generates an ERROR, WARNING or INFO message about
        a change in the jars being compared, there is an associated
        message reference code. This document contains an explanation
        of the meaning of that message which may contain information
        which could not be fitted into the brief message summary.
      </p>
      <p>
        Messages are separated into three severity levels:
        <ul>
          <li>ERROR</li>
          <li>WARNING</li>
          <li>INFO</li>
        </ul>
      </p>
      <p>
        Errors come in two flavours: 
        <ul>
          <li>
            Link-time failures, where an exception will be thrown as soon
            as code compiled against an old version of a class and the
            new version of the class are loaded into the same classloader
            hierarchy.
          </li>
          <li>
            Run-time failures, where an exception is thrown when code
            compiled against the old version of a class attempts to call a 
            method on a new version of the class, or vice versa.
          </li>
        </ul>
        <p>
          Clirr reports "errors" for cases where it is <i>possible</i>
          to get a run-time failure. Whether one actually occurs can
          depend upon the way the library is called, ie changes reported
          as an error may in fact work when used as long as the patterns
          of use of the library do not trigger the failure situation.
        </p>
      </p>
      <p>
        Warnings are issued for situations where no link or runtime
        exception will occur, but where the application may behave
        unexpectedly due to the changes that have occurred.
      </p>
      <p>
        Information messages provide users with information about
        new features which have been added without breaking backward
        compatibility in any way.
      </p>
      <p>
        When using clirr to report on changes to items which have
        private or package scope, these changes are always reported
        as INFO level changes, never WARNING or ERROR level. This allows
        users of clirr to generate "change reports" at a level suitable for
        developers without having some of those changes marked (irrelevantly) 
        as binary incompatibilities.
      </p>
      <p>
        There can never be binary incompatibilities for changes to private 
        classes, methods or fields as that access can only occur from 
        within the same class (ie the same compilation unit). 
      </p>
      <p>
        Clirr does not report binary incompatibility WARNINGs or ERRORs for 
        package-scoped items either, because java packages are intended to be 
        "release units", ie all classes within a package are compiled together 
        (ensuring compatibility) then released as a unit. The only time that 
        package-scope incompatibilities could possibly be an issue is when 
        users of a library write their own classes using a package declaration 
        belonging to some external library, or when a subset of updated classes 
        (eg a single class) from a package is used to override certain classes 
        from a previous release of the library. Both of these situations are 
        considered very poor practice by java programming convention. 
      </p>
      <p>
        In the following sections, the term "old" is used to refer to
        a class, interface, method or field from the set of jars which
        represent the old/previous/original/baseline version of the
        library being inspected. The term "new" is used to refer to
        a class, interface, method or field from the set of jars
        which represent the new/current/latest version of the library
        being inspected.
      </p>
      <p>
        In the following sections, the term "type" is used to refer to
        something which may be either a class or interface. 
      </p>
    </section>
    <section name="Messages">
      <section name="1000 - Increased visibility of class">
        <p>Severity: <code>INFO</code></p>
        <p>
          The specified type exists in both versions, but its declared access 
          specifier has changed to relax restrictions on what other code can 
          access it.
        </p>
        <p>
          Top-level types (ie those which are not nested within another class) 
          may only have "package" or "public" accessibility. Nested types can
          take on any of the four available accessibility values.
        </p>
        <p>
          Regardless of whether the object is top-level or nested, a change
          in accessibility from left-to-right of the sequence
          private->package->protected->public always ensures that all code which
          could previously access that type can still access that type.
          Therefore such a change is always binary and source-code compatible.
        </p>
        <p>
          Note that the declaration "protected" provides access to <i>both</i>
          code derived from the type <i>and</i> to code within the same package,
          ie "protected" accessibility also implies package accessibility.
        </p>
      </section>

      <section name="1001 - Decreased visibility of class">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The specified type exists in both versions, but its declared access 
          specifier has changed to tighten the restrictions on what other code 
          may access it. 
        </p>
        <p>
          Top-level types (ie those which are not nested within another class) 
          may only have "package" or "public" accessibility. Nested types can 
          take on any of the four available accessibility values.
        </p>
        <p>
          Regardless of whether the type is top-level or nested, a change
          in accessibility from left-to-right of the sequence
          public->protected->package->private may cause existing code which
          could previously access the type to no longer be able to do so. 
        </p>
        <p>
          Section 13.4.3 of the java language specification states explicitly 
          that an IllegalAccessError should occur if a pre-existing binary tries
          to access a type when the type accessibility has been changed to
          something that would cause a compile-time error. However this does
          not appear to be enforced in practice, at least in current JVMs. 
          Nevertheless this <i>should</i> be an error, and so clirr reports 
          this change as a binary-compatibility ERROR.
        </p>
      </section>

      <section name="2000 - Changed from class to interface">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The specified class has become an interface in the new version.
          This change is always a binary and source-code incompatibility, for 
          obvious reasons.
        </p>
      </section>

      <section name="2001 - Changed from interface to class">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The specified interface has become an class in the new version.
          This change is always a binary and source-code incompatibility, for 
          obvious reasons.
        </p>
      </section>

      <section name="3001 - Removed final modifier from class">
        <p>Severity: <code>INFO</code></p>
        <p>
          The specified class was declared final in the old version, but is
          no longer final in the new version.
        </p>
      </section>

      <section name="3002 - Added final modifier to effectively final class">
        <p>Severity: <code>INFO</code></p>
        <p>
          The specified class was not declared final in the old version, but is
          now declared final. Normally, this would be an incompatibility
          because pre-existing derived classes would no longer be valid when
          used with the new version of this class. However in this case
          the old class version had no public or protected constructors,
          so it was not possible for any derived classes to exist even for
          the old version of the library. Changing such a class to final
          therefore can not break any existing code.
        </p>
      </section>

      <section name="3003 - Added final modifier to class">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The specified class was not declared final in the old version, but is
          now declared final. Any pre-existing classes which were declared
          as subclasses of this class will therefore not be valid with the
          new version of the library.
         </p>
         <p>
          A VerifyError is thrown by the classloader when an attempt is made to 
          load a subclass of a final class.
         </p>
         <p>
          Note that a class Y is loaded by the standard classloader only when
          the first attempt is made to create an instance of Y, or to directly
          reference the Class object for class Y. If some other class X has 
          class Y as a declared member, or as a parameter to some method, then 
          loading class X does <em>not</em> cause class Y to be loaded.
         </p>
      </section>

      <section name="3004 - Removed abstract modifier from class">
        <p>Severity: <code>INFO</code></p>
        <p>
          The old version of this class was declared to be an abstract
          class. The new version is not abstract, allowing users to
          create instances of the class. 
        </p>
      </section>

      <section name="3005 - Added abstract modifier to class">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The old version of this class was not declared to be abstract.
          The new version is abstract. Pre-existing code which creates
          instances of this class is no longer valid with the new version.
        </p>
      </section>

      <section name="4000 - Added interface to the set of implemented interfaces">
        <p>Severity: <code>INFO</code></p>
        <p>
          The new version of the type now implements an additional interface. 
          This does not invalidate any existing code (source or binary), and is 
          a completely backward-compatible change.
        </p>
        <p>
          Note that this message can be reported without any change occurring
          in the specified type; a change to the set of interfaces supported by 
          a type will cause this message to be reported for every descendant
          of that type.
        </p>
      </section>

      <section name="4001 - Removed interface from the set of implemented interfaces">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The old version of this type declared that it implemented an interface 
          which the new class or interface does not. Existing code which 
          explicitly or implicitly casts objects of this type to the now missing 
          interface is no longer valid.
        </p>
        <p>
          Note that this message can be reported without any change occurring
          in the specified type; a change to the set of interfaces supported by 
          a type will cause this message to be reported for every descendant
          of that type.
        </p>
      </section>

      <section name="5000 - Added class to the set of superclasses">
        <p>Severity: <code>INFO or WARNING</code></p>
        <p>
          The new version of the class has a class in its inheritance hierarchy
          which the old version did not, either because its direct parent is 
          now a different class, or because one of its parent classes has 
          changed its inheritance hierarchy.
        </p>
        <p>
         If the specified class has java.lang.Throwable as an ancestor, then 
         this change is reported as a WARNING, because this class change may 
         change the exception-catching behaviour of programs that use this class.
        </p>
        <p>
          Note that this message can be reported without any change occurring
          in the specified class; a change to the set of superclasses of an
          ancestor class will cause this message to be reported for every 
          descendant class.
        </p>
      </section>

      <section name="5001 - Removed class from the set of superclasses">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The old version of this class has a class in its inheritance hierarchy
          which the new version does not, either because its direct parent
          is now a different class, or because one of its parent classes has
          changed its inheritance hierarchy.
        </p>
        <p>
          Existing code which explicitly or implicitly casts objects of this 
          type to the now missing class type is no longer valid.
        </p>
        <p>
          Note that this message can be reported without any change occurring
          in the specified class; a change to the set of superclasses of an
          ancestor class will cause this message to be reported for every 
          descendent class.
        </p>
        <p>
          Note also that if this class has Throwable in its ancestry, then
          the class hierarchy change can also cause changes in the
          exception-catching behaviour of programs using this class.
        </p>
      </section>

      <section name="6000 - Added field">
        <p>Severity: <code>INFO</code></p>
        <p>
          The new class has an additional static or instance member. This
          change is completely backwards-compatible.
        </p>
      </section>

      <section name="6001 - Removed field">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The new class has removed a field present in the old version.
          Pre-existing code which directly accesses that field will no longer 
          be valid.
        </p>
      </section>

      <section name="6002 - Value of field no longer a compile-time constant">
        <p>Severity: <code>WARNING</code></p>
        <p>
          Code compiled against the old version of the class was permitted to
          "inline" the value of this field because it was a compile-time constant.
          Therefore, existing binary code will continue to use the old value of
          this field, instead of the new value (which cannot be inlined).
        </p>
      </section>

      <section name="6003 - Value of compile-time constant has changed">
        <p>Severity: <code>WARNING</code></p>
        <p>
          Code compiled against the old version of the class was permitted to
          "inline" the value of this field because it was a compile-time constant.
          Therefore, existing binary code will continue to use the old value of
          this field, instead of the new value.
        </p>
      </section>

      <section name="6004 - Field type changed">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The type associated with the specified static or instance member
          of the specified class has changed. Pre-existing code which directly 
          accesses that field may no longer be valid, and therefore this is
          an incompatible change.
        </p>
      </section>

      <section name="6005 - Field now non-final">
        <p>Severity: <code>INFO</code></p>
        <p>
          The field was previously final, and is no longer final. This means
          that the field value can now be modified during the lifetime of
          the class or instance.
        </p>
        <p>
          Whether a value in a field could previously be "inlined" into
          other classes is an issue addressed by messages 6002 and 6003,
          not this message.
        </p>
      </section>

      <section name="6006 - Field now final">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The field can no longer be modified during the lifetime of the
          class or instance. Code which previously modified this field is
          therefore no longer valid.
        </p>
      </section>

      <section name="6007 - Field now non-static">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The field is now an instance variable rather than a class variable.
          Code which previously accessed this field via the Class rather
          than an instance of the class is no longer valid.
        </p>
      </section>

      <section name="6008 - Field now static">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The field is now a class variable rather than an instance variable.
        </p>
        <p>
          For some reason (presumably internal implementation issues), the
          java standard declares that this change is not binary-compatible,
          and that an IncompatibleClassChangeError will be thrown if code
          compiled against the "old" version of a class is used together with
          a "new" version for which a field is now static.
        </p>
        <p>
          Because source code is permitted to access class variables via
          instances of that class, this is expected to be a source-code
          compatible change. However currently CLIRR reports this as an
          ERROR for source-code compatibility too.
        </p>
      </section>

      <section name="6009 - Field More Accessible">
        <p>Severity: <code>INFO</code></p>
        <p>
          In the new version, the specified field is accessible to more
          code than it was previously. 
        </p>
      </section>

      <section name="6010 - Field Less Accessible">
        <p>Severity: <code>ERROR</code></p>
        <p>
          In the new version, the specified field is accessible to less
          code than it was previously. Therefore existing code may no longer
          be valid.
        </p>
      </section>

      <section name="6011 - Removed Constant Field">
        <p>Binary Severity: <code>WARNING</code></p>
        <p>Source Severity: <code>ERROR</code></p>
        <p>
          The new class has removed a field present in the old version.
          Pre-existing source code which directly accesses that field will no 
          longer be valid.
        </p>
        <p>
          Previously, however, the field was final and was initialised with a 
          constant value. Therefore code compiled against the previous version 
          of the class will have inlined this constant and will continue to 
          work, using the previous value of this field. A warning is issued
          as this is often not desirable behaviour. However it is not a
          binary incompatibility.
        </p>
      </section>

      <section name="7000 - Method now in Superclass">
        <p>Severity: <code>INFO</code></p>
        <p>
          The old class had a method named X. The new class no longer has this 
          method, but a parent class does define this method, so no binary or 
          source incompatibility has occurred.
        </p>
        <p>
          Note that this change may have the effect of forcing the new
          class to become 'abstract'. If this is the case, then this change
          is reported separately.
        </p>
      </section>

      <section name="7001 - Method now in Interface">
        <p>Severity: <code>INFO</code></p>
        <p>
          The old class or interface previously had a method named X. The 
          new class or interface no longer has this method, but a parent 
          interface does define this method, so no binary or source 
          incompatibility has occurred.
        </p>
        <p>
          Note that this change may have the effect of forcing the new
          class to become 'abstract'. If this is the case, then this change
          is reported separately.
        </p>
      </section>

      <section name="7002 - Method Removed">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The old class or interface had a method named X. The new class or
          interface no longer has this method, and this method is not defined 
          on any parent class or interface.
        </p>
        <p>
          Whether an error actually occurs at runtime for this change depends
          on usage patterns. The modified class can be used with existing
          code as long as that existing code does not attempt to call the
          removed method. If a call to the missing method is made, then
          a NoSuchMethodError exception is generated when the method
          invocation occurs.
        </p>
      </section>

      <section name="7003 - Method Overide Removed">
        <p>Severity: <code>INFO</code></p>
        <p>
          The specified method on the old class or interface was overriding an
          inherited definition. The new class or interface no longer has this 
          method explicitly declared on it, but it still inherits a definition
          so there is no binary incompatibility.
        </p>
      </section>

      <section name="7004 - Method Argument Count Changed">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The specified method has had arguments added or removed. This means
          that code which previously invoked it will no longer invoke the same
          method.
        </p>
        <p>
          If there is an inherited method definition with the old prototype,
          then there is no binary incompatibility; code which was compiled
          against the old version of this class will now invoke the inherited
          implementation. <em>In this situation, clirr should output an INFO
          message rather than an error. However at the current date, clirr does 
          not check for this situation</em>.
        </p>
        <p>
          If there is no inherited method definition with the old prototype,
          then the change is a binary incompatibility.
        </p>
      </section>

      <section name="7005 - Method Argument Type changed">
        <p>Binary Severity: <code>INFO or ERROR</code></p>
        <p>Source Severity: <code>ERROR</code></p>
        <p>
          The specified method has had the type of one or more of its arguments
          modified. This means that code compiled against the old version of
          the class will no longer invoke the same method. However exactly
          the same old source code, when compiled against the new class
          version <i>may</i> invoke this method if the argument types are
          assignment-compatible.
        </p>
        <p>
          If there is an inherited method definition with the old prototype,
          then there is no binary incompatibility; code which was compiled
          against the old version of this class will now invoke the inherited
          implementation.
          <em>At the current date, clirr does not check for this situation</em>.
        </p>
        <p>
          If there is no inherited method definition with the old prototype,
          then the change is a binary incompatibility.
        </p>
        <p>
          If the parameter types changed were all changed to <i>supertypes</i>
          of their previous declared types, or for primitive parameter types if
          they were changed to "larger" types in every case, then the new
          code is <i>source-code-compatible</i> with the previous release
          even if it is not binary-compatible. Note that in this situation,
          recompiling code which uses the library may change its behaviour
          from calling an inherited method to calling a method on the class
          which has a slightly different prototype.
          <em>At the current date, clirr does not check for this situation</em>.
        </p>
      </section>

      <section name="7006 - Method Return Type changed">
        <p>Binary Severity: <code>ERROR</code></p>
        <p>Source Severity: <code>INFO or ERROR</code></p>
        <p>
          The specified method has had its declared return type changed. Whether
          a problem actually occurs at runtime when using code compiled against
          the old version of this library depends upon usage patterns. Old
          code may call other methods on this class. However any attempt to
          call the method whose return type has changed will result in a
          NoSuchMethodError being thrown when the method is invoked, because
          the return type is part of the "method signature".
        </p>
        <p>
          The change is <i>source-code-compatible</i> if and only if the new 
          return type is <i>assignable to</i> the old return type. This means 
          that:
          <ul>
            <li>
              if the old return type was a primitive type, then the new return 
              type must be <i>narrower</i> than the old type.
            </li>
            <li>
              if the old return type was an interface, then the new return type
              must be a class or interface which implements the old return type. 
            </li>
            <li>
              if the old return type was a class, then the new return type must
              be a subclass of the previously returned type.
            </li>
          </ul>
          Clirr does not currently check for source-code compatibility for
          changes in method return types; currently these are simply
          reported as an ERROR.
        </p>
      </section>

      <section name="7007 - Method has been Deprecated">
        <p>Severity: <code>INFO</code></p>
        <p>
          The specified method has been declared as "deprecated". This is
          always a binary-compatible change as well as a source-code-compatible
          change.
        </p>
      </section>

      <section name="7008 - Method has been Undeprecated">
        <p>Severity: <code>INFO</code></p>
        <p>
          The specified method was declared "deprecated" in the previous 
          version, but is no longer deprecated in the current release. While
          slightly unusual, this is permitted. This change is always a
          binary-compatible change as well as a source-code-compatible change.
        </p>
      </section>

      <section name="7009 - Method is now Less Accessible">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The access permissions associated with the specified method have
          been tightened to permit less user code to access the method.
        </p>
        <p>
          Whether this change is a source-code compatibility issue or not
          depends upon patterns of usage.
        </p>
        <p>
          This change <i>should</i> be a binary incompatibility. Note,
          however, that current JVMs do not validate this. Code compiled
          against a previous version of a class can successfully invoke
          methods for which they no longer have access rights. Nevertheless,
          the java language specification states that this is an error, so
          clirr reports this change as a binary incompatibility.
        </p>
      </section>

      <section name="7010 - Method is now More Accessible">
        <p>Severity: <code>INFO</code></p>
        <p>
          The access permissions associated with the specified method have
          been loosened to permit more user code to access the method. This
          is always a binary and source-code compatible change.
        </p>
      </section>

      <section name="7011 - Method Added">
        <p>Severity: <code>INFO</code></p>
        <p>
          A non-abstract method has been added to the specified class. This is
          always a binary-compatible change.
        </p>
        <p>
          It is also a source-code compatible change.
        </p>
        <p>
         Q: if the new method overrides an inherited one, then which version
         does code compiled against the old library invoke?
        </p>
      </section>

      <section name="7012 - Method Added to Interface">
        <p>Binary Severity: <code>ERROR</code></p>
        <p>Source Severity: <code>ERROR</code></p>
        <p>
          A method declaration has been added to the specified interface. This
          is always reported as a binary-compatibility error, but in practice
          the changed class <i>might</i> be used successfully with code compiled
          against the old interface depending upon usage patterns.
        </p>
        <p>
          Old code which invokes methods upon code compiled against the new
          (expanded) interface will continue to work without issues. And old 
          code which implements the old version of the interface will also 
          continue to work correctly as long as no code attempts to invoke
          any of the newly-added methods against that instance. But code which
          (validly) invokes one of the new methods in the interface against
          an object which implements only the old version of the interface will
          cause an AbstractMethodError to be thrown at the time the method
          invocation is attempted.
        </p>
        <p>
          Adding a method to an interface is always reported as an ERROR,
          because classes that implement that interface must now be modified
          to implement the declared method.
        </p>
      </section>

      <section name="7013 - Abstract Method Added to Class">
        <p>Binary Severity: <code>ERROR</code></p>
        <p>Source Severity: <code>ERROR</code></p>
        <p>
          An abstract method declaration has been added to the specified class.
          This is always reported as a binary-compatibility error, but in practice
          the changed class <i>might</i> be used successfully with code compiled
          against the old class depending upon usage patterns.
        </p>
        <p>
          If instances of objects compiled against the old class are created,
          then their methods can be invoked without problems. But if the
          newly-added abstract method is ever invoked, then an AbstractMethodError
          is thrown at the time the method invocation is attempted.
        </p>
      </section>

      <section name="7014 - Method now final">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The method was previously non-final, and is now final.
          Subclasses of this class will no longer compile or
          run.
        </p>

        <p>
          When the old class containig this method was final
          (explicitly or by only providing private constructors) 
          then subclasses cannot exist. Clirr currently does not check
          for this situation, so this will raise a false alarm
          in some corner cases.
        </p>
      </section>

      <section name="7015 - Method now non-final">
        <p>Severity: <code>INFO</code></p>
        <p>
          The method was previously final, and is now non-final. This is
          always a binary-compatible change.
        </p>
      </section>

      <section name="8000 - Class Added">
        <p>Severity: <code>INFO</code></p>
        <p>
          The new version of the library has a class which was not present
          in the old version.
        </p>
      </section>

      <section name="8001 - Class Removed">
        <p>Severity: <code>ERROR</code></p>
        <p>
          The new version of the library no longer contains the specified
          class.
        </p>
      </section>

    </section>
  </body>
</document>
