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Hide JDTCompilerAdapter from org.eclipse.jdt.core class loader
See README.md why the JDTCompilerAdapter classloading is not trivial. TL;DR: OSGI classloader from org.eclipse.jdt.core bundle should not find JDTCompilerAdapter class (!) so the ant started from Eclipse can find it inside jdtCompilerAdapter.jar contributed by ... org.eclipse.jdt.core bundle. Fixes eclipse-pde/eclipse.pde#419
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org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/core/README.md
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| ## Notes for maintainers of the JDTCompilerAdapter (ant support) in ECJ. | ||
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| The org.eclipse.jdt.core.JDTCompilerAdapter class can be used by PDE/ant | ||
| via jdtCompilerAdapter.jar if running ant tasks from inside Eclipse or by ant | ||
| directly (by adding org.eclipse.jdt.core.compiler.batch jar to ant classpath) | ||
| if running in a separated VM without OSGI. | ||
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| ### Standalone ant use | ||
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| Using JDTCompilerAdapter class from standalone ant is straightforward - the | ||
| org.eclipse.jdt.core.compiler.batch jar should be added to the ant javac classpath | ||
| in some way, like for example here: | ||
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| ``` | ||
| <javac destdir="${temp.folder}/@dot.bin" failonerror="${javacFailOnError}" verbose="${javacVerbose}" | ||
| debug="${javacDebugInfo}" includeAntRuntime="no" source="${javacSource}" target="${javacTarget}" encoding="UTF-8"> | ||
| <compilerarg line="${compilerArg}" compiler="${build.compiler}"/> | ||
| <classpath refid="@dot.classpath" /> | ||
| <src path="src/" /> | ||
| <compilerarg line="-properties '.settings/org.eclipse.jdt.core.prefs'" compiler="org.eclipse.jdt.core.JDTCompilerAdapter" /> | ||
| <compilerarg line="-log '${temp.folder}/@dot.binnull'" compiler="org.eclipse.jdt.core.JDTCompilerAdapter" /> | ||
| <compilerarg line="-g -showversion -encoding UTF-8 -preserveAllLocals | ||
| -enableJavadoc -nowarn:[src-gen] -nowarn:[.src-gen] -time" compiler="org.eclipse.jdt.core.JDTCompilerAdapter" /> | ||
| <compilerclasspath> | ||
| <pathelement path="${ecjBatchCompilerJarLocation}"/> | ||
| </compilerclasspath> | ||
| </javac> | ||
| ``` | ||
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| ### Eclipse (OSGI) embedded ant use | ||
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| Using JDTCompilerAdapter class from ant started from OSGI container (Eclipse) is | ||
| not trivial and has few pitfalls. | ||
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| First of all, org.eclipse.jdt.core.JDTCompilerAdapter class is packaged into a dedicated | ||
| jdtCompilerAdapter.jar that is **not** part of org.eclipse.jdt.core.compiler.batch bundle! | ||
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| This jdtCompilerAdapter.jar is packaged **inside** org.eclipse.jdt.core bundle for some obscure reasons | ||
| and is extracted at runtime in org.eclipse.ant.internal.ui.datatransfer.BuildFileCreator.addInitEclipseCompiler(). | ||
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| Second most surprising point is: if the JDTCompilerAdapter code runs in OSGI environment, in order | ||
| to be properly loaded by ant which is started by PDE/ant.core bundle via | ||
| org.eclipse.ant.core.AntRunner.run(Object), the org.eclipse.jdt.core.JDTCompilerAdapter class | ||
| should **NOT** be found by classloaders of org.eclipse.jdt.core or org.eclipse.jdt.core.compiler.batch bundles! | ||
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| The reason is, that the JDTCompilerAdapter class itself needs ant classes for class initialization, | ||
| but that must be **same** ant classes loaded by org.eclipse.ant.internal.core.AntClassLoader. | ||
| If the ant classes needed by JDTCompilerAdapter class are loaded by OSGI bundle loaders, | ||
| ant engine can't use them (even if they are coming from same jar)! From Java | ||
| runtime point of view they are different because loaded by a different classloader. | ||
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| Therefore the code in org.eclipse.ant.internal.core.AntClassLoader.loadClassPlugins(String) | ||
| that goes over possible **bundle** classloaders that contribute ant tasks for Eclipse, | ||
| **must** fail to load org.eclipse.jdt.core.JDTCompilerAdapter class and return *null*! | ||
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| This is surprising at least, and has some non trivial consequences. | ||
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| The main one is: org.eclipse.jdt.core.JDTCompilerAdapter should be "not visible" | ||
| for default OSGI class loading from org.eclipse.jdt.core bundle that contributes | ||
| extra ant classpath entry jdtCompilerAdapter.jar, | ||
| and therefore org.eclipse.jdt.core package should be **not exported** in MANIFEST.MF | ||
| by org.eclipse.jdt.core.compiler.batch! | ||
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| After failing to load JDTCompilerAdapter class in AntClassLoader.loadClassPlugins(String) | ||
| the AntClassLoader code goes to the parent classloader (which is URLClassLoader) | ||
| and the parent walks over **extra** ant classpath entries contributed by plugins | ||
| via "org.eclipse.ant.core.extraClasspathEntries" extension point (see | ||
| org.eclipse.ant.core.AntCorePreferences.computeDefaultExtraClasspathEntries(List)). | ||
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| The URLClassLoader finally finds and loads JDTCompilerAdapter in jdtCompilerAdapter.jar | ||
| contributed by org.eclipse.jdt.core as an extra ant classpath entry | ||
| (with all the required ant classes found by ant own classloader)! | ||
| The URLClassLoader (AntClassLoader) should be initialized with core ant libraries at the beginning, so | ||
| it has no issues to satisfy all JDTCompilerAdapter ant dependencies. |