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From "Uwe Schindler (JIRA)" <j...@apache.org>
Subject [jira] [Created] (LUCENE-4808) Add workaround for a JDK 8 "class library bug" which is still under discussion any may *not* be fixed
Date Fri, 01 Mar 2013 10:51:13 GMT
Uwe Schindler created LUCENE-4808:
-------------------------------------

             Summary: Add workaround for a JDK 8 "class library bug" which is still under
discussion any may *not* be fixed
                 Key: LUCENE-4808
                 URL: https://issues.apache.org/jira/browse/LUCENE-4808
             Project: Lucene - Core
          Issue Type: Bug
          Components: general/build
    Affects Versions: 4.1, 5.0
            Reporter: Uwe Schindler
            Assignee: Uwe Schindler


With JDK8 build 78 there was introduced a backwards compatibility regression which may not
be fixed until release, because Oracle is possibly accepting this backwards break regarding
cross-compilation to JDK6.

The full thread on the OpenJDK mailing list: [http://mail.openjdk.java.net/pipermail/compiler-dev/2013-February/005737.html]
(continues in next month: http://mail.openjdk.java.net/pipermail/compiler-dev/2013-March/005748.html)

*In short:* JDK 8 adds so called default implementations on interfaces (means you can add
a new method to an interface and provide a "default" implementation, so code implementing
this interface is not required to implement the new method. This is really cool and would
also help Lucene to make use of Interfaces instead of abstract classes which don't allow polymorphism).

In Lucene we are still compatible with Java 7 and Java 6. So like millions of other open source
projects, we use "-source 1.6 -target 1.6" to produce class files which are Java 1.6 conform,
although you use a newer JDK to compile. Of course this approach has problem (e.g. if you
use older new methods, not available in earliert JDKs). Because of this we must at least compile
Lucene with a JDK 1.6 legacy JDK and also release the class files with this version. For 3.6,
the RM has to also install JDK 1.5 (which makes it impossible to do this on Mac). So -source/-target
is a alternative to at least produce 1.6/1.5 compliant classes. According to Oracle, this
is *not* the correct way to do this: Oracle says, you have to use: -source, -target and -Xbootclasspath
to really crosscompile -> and the last thing is what breaks here. To correctly set the
bootclasspath, you need to have an older JDK installed or you should be able to at least download
it from maven (which is not available to my knowledge).

The problem with JDK8 is now: If you compile with -source/-target but not the bootclasspath,
it happens that the compiler does no longer understand new JDK8 class file structures in the
new rt.jar, so producing compile failures. In the case of this bug: AnnotatedElement#isAnnotationPresent()
exists since Java 1.5 in the interface, but all implementing classes have almost the same
implementation: "return getAnnotation(..) != null;". So the designers of the class library
decided to move that method as so called default method into the interface itsself, removing
code duplication. If you then compile code with "-source 1.6 -target 1.6" using that method,
the javac compier does not know about the new default method feature and simply says: "Method
not found in java.lang.Class":

{noformat}
    [javac] Compiling 113 source files to C:\Users\Uwe Schindler\Projects\lucene\trunk-lusolr3\lucene\build\test-framework\classes\java
    [javac] C:\Users\Uwe Schindler\Projects\lucene\trunk-lusolr3\lucene\test-framework\src\java\org\apache\lucene\util\TestRuleSetupAndRestoreClassEnv.java:134:
error: cannot find symbol
    [javac]     if (targetClass.isAnnotationPresent(SuppressCodecs.class)) {
    [javac]                    ^
    [javac]   symbol:   method isAnnotationPresent(Class<SuppressCodecs>)
    [javac]   location: variable targetClass of type Class<?>
    [javac] Note: Some input files use or override a deprecated API.
    [javac] Note: Recompile with -Xlint:deprecation for details.
    [javac] 1 error
{noformat}

But until the Oracle people have a good workaround (I suggested to still implement the method
on the implementation classes like Class/Method/... but delegate to the interface's default
impl), we can quickly commit a replacement of this broken method call by (getAnnotation(..)
!= null). I want to do this, so we can enable jenkins builds with recent JDK 8 again.

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