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116 lines
4.8 KiB
Markdown
116 lines
4.8 KiB
Markdown
---
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layout: developer-doc
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title: Java 11
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category: infrastructure
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tags: [infrastructure, build, java11]
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order: 2
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---
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# Migrating to Java 11
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JDK 11 will be supported longer than JDK 8 that we currently use and it adds new
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features that could improve performance. Moreover, we want to be compliant to
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the Java Platform Module System, as all future versions of the JDK will rely on
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it. Thus, we have moved to using Graal builds for Java 11.
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<!-- MarkdownTOC levels="2,3" autolink="true" -->
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- [Migration Progress](#migration-progress)
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- [Build Configuration](#build-configuration)
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- [Testing](#testing)
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- [Benchmarks](#benchmarks)
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- [Problems](#problems)
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- [IllegalAccessError](#illegalaccesserror)
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<!-- /MarkdownTOC -->
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## Migration Progress
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The overall steps of the migration and their status are outlined in this
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section.
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### Build Configuration
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The option `-XX:-UseJVMCIClassLoader` is deprecated in Java 11 and has been
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removed from the test configuration.
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The JVM running sbt must have `--upgrade-module-path=lib/truffle-api.jar` added
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as an option and the build tool must ensure that the `truffle-api.jar` is copied
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from the Maven repository to the `lib/` directory before the `runtime` project
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is compiled. Section [IllegalAccessError](#illegalaccesserror) explains why this
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is necessary and [Bootstrapping](./sbt.md#bootstrapping) explains the tasks that
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help with this process.
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### Testing
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All tests are passing.
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To make sure `runtime` tests can be run both with `test` and `testOnly`,
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`--upgrade-module-path=<path-to-truffle-api.jar>` has to be added to
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`javaOptions`. It is important to note that the tests may be ran in a working
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directory different than the project root, so an absolute path should be used.
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### Benchmarks
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Initially there were some regressions found in the benchmarks, but further
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investigation revealed this was caused by some issues in the methodology of how
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the JMH benchmarks were implemented. There are plans to rewrite these
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benchmarks.
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Benchmarks in pure Enso are currently more meaningful. They yield comparable
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results with Java 11 being slightly faster.
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## Problems
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The problems that were encountered when doing the migration.
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### IllegalAccessError
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As described in [Build tools](sbt.md#incremental-compilation), to allow for
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incremental compilation, zinc has to analyse dependencies between various files.
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As our project uses both Scala and Java code, dependencies between files in both
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languages have to be detected.
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To achieve that, after compiling Java classes with `javac`, zinc loads the
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resulting classes and analyses their APIs to find what other classes they depend
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on. Based on this information it can figure out what files need to be recompiled
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on any changes.
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The sbt process running zinc code tries to load our classes, but it fails for
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some of them with errors like:
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```
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java.lang.IllegalAccessError: superinterface check failed: class com.oracle.truffle.sl.runtime.SLFunction (in unnamed module @0x7590b48c) cannot access class com.oracle.truffle.api.interop.TruffleObject (in module org.graalvm.truffle) because module org.graalvm.truffle does not export com.oracle.truffle.api.interop to unnamed module @0x7590b48c
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```
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The Truffle API does not export its packages for security reasons and it uses
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some custom mechanisms when loading the language runtime. However zinc is not
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aware of these mechanisms and just directly loads the `.class` files, resulting
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in errors.
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Some of these errors are caught and instead of failing, simply a warning is
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printed. Others are not detected where zinc expects them, but they fail later,
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crashing the compilation process. All of them are problematic though, because
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they mean that zinc is not able to read dependencies of the affected files. This
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harms incremental compilation (as some dependencies are not detected it might be
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necessary to do a clean and full recompilation when changing these files).
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#### Solution
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We want to make the ClassLoader read these class files without errors. For that
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we need to ensure it has permissions to load the Truffle modules.
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Truffle API is distributed in two ways. The distribution included in the Graal
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runtime (the one that is picked up by sbt by default) exports the required APIs
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only to its own modules, so they are not available for us (thus the errors).
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This is to ensure better security (to disallow language users introspecting the
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VM internals). However, there is a second Truffle distribution on Maven that is
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to be used for development only and that version exports the necessary APIs to
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all packages.
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We need to ensure that the sbt process doing the compilation uses the Maven
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version of Truffle, so that it does not complain about the illegal accesses. To
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achieve that, we need to add the option
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`--upgrade-module-path=lib/truffle-api.jar` to the JVM running sbt and ensure
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that the Truffle JAR from maven is copied to the `lib/` directory.
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