Frgaal

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Revision as of 07:07, 23 April 2022 by JaroslavTulach (Talk | contribs)
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I love Frgaal and you'll love it too: Wouldn't it be great to have records and other latest Java language features available on your old JDKs? As old as JDK8? Frgaal allows you to do it. There is a Maven plugin, integration with Gradle, a way to use it from command line... read more at http://frgaal.org


Can I use Frgaal via ToolProvider API?

Let's imagine we have a following code (that invokes the JavaCompiler via ToolProvider for a custom CODE snippet defining Record with main method: 

import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
 
class ModernJavaCompiler {
  private static final String CODE = ""
          + "record R (String name) {\n"
          + "  public static void main(String... args) {\n"
          + "    System.out.println(new R(\"Hello World!\"));\n"
          + "  }\n"
          + "}\n";
 
  public static void main(String... args) throws Exception {
      JavaCompiler compiler = javax.tools.ToolProvider.getSystemJavaCompiler();
      List<String> arguments = Arrays.asList(args);
      List<JavaFileObject> sources = Collections.singletonList(new Src(CODE));
      JavaCompiler.CompilationTask task = compiler.getTask(null, null, null, arguments, null, sources);
      Boolean res = task.call();
      if (!Boolean.TRUE.equals(res)) {
          throw new IllegalStateException("Compilation failed: " + res);
      } else {
          System.out.println("Compiled OK");
      }
  }
 
  static class Src extends SimpleJavaFileObject {
        private final String code;
 
        public Src(String code) throws URISyntaxException {
            super(new URI("memory://Code.java"), Kind.SOURCE);
            this.code = code;
        }
 
        @Override
        public String getName() {
            return "Code.java";
        }
 
 
        @Override
        public CharSequence getCharContent(boolean bln) throws IOException {
            return code;
        }
  }
}

This is a regular Java source file that can be compiled using standard Javac compiler: 

$ javac -source 8 -target 8 ModernJavaCompiler.java

The ModernJavaCompiler source code then uses JavaCompiler on its own to generate class R. It can be executed JDK-17 as 

$ /jdk-17/bin/java ModernJavaCompiler
Compiled OK
$ /jdk-17/bin/java R                                                                                                                     
R[name=Hello World!]

However executing the same steps with JDK-11 is going to fail as the Javac from JDK-11 doesn't support record keyword: 

$ /jdk11/bin/java ModernJavaCompiler
Code.java:1: error: class, interface, or enum expected
record R (String name) {
^

Can we fix that? Sure...

The idea would be to replace the standard "compiler tool" with the one coming from the frgaal project. Let's try to disable the regular compiler first. There is an option --limit-modules in JDK-11+ and the idea is to use it to disable standard JavaC: 

$ /jdk11/bin/java --limit-modules java.base ModernJavaCompiler
Exception in thread "main" java.lang.NoClassDefFoundError: javax/tools/ToolProvider

Great! Disabling works. The "compiler tool" is completely missing. Now there is a time to inject Frgaal instead. Following script shows how to get it all working: 

$ mkdir -p META-INF/services
$ echo com.sun.tools.javac.api.JavacTool > META-INF/services/javax.tools.JavaCompiler
$ curl -o compiler.jar https://repo1.maven.org/maven2/org/frgaal/compiler/18.0.0/compiler-18.0.0.jar
$ /jdk-11/bin/java --limit-modules java.base,jdk.zipfs -cp compiler.jar:. ModernJavaCompiler
warning: [options] system modules path not set in conjunction with -source 17
warning: No output directory specified, cannot perform multi-release compilation
2 warnings
Compiled OK
~/bin/jdk-11/bin/java R
R[name=Hello World!]

Hey, using record on JDK-11!

Having record (and other JDK-18 language features) available on old JDKs is great, isn't it? All that is necessary is to provide extra ServiceLoader registration in META-INF/services path on the class path. Then the Frgaal compiler gets picked up and is used for compilation even on older JDKs.


Embed Frgaal into your own programs and use the latest Java features on any JDK!

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