path: root/src/java_tools/buildjar/java/com/google/devtools/build/java/bazel/BazelJavaCompiler.java

// Copyright 2015 The Bazel Authors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package com.google.devtools.build.java.bazel;

import com.google.common.collect.ImmutableList;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.Writer;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.nio.charset.Charset;
import java.util.List;
import java.util.Locale;
import java.util.Set;
import javax.lang.model.SourceVersion;
import javax.tools.DiagnosticListener;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileManager;
import javax.tools.JavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.StandardLocation;

/**
 * Provides a {@link JavaCompiler} that has behavior as similar as possible to the java compiler
 * provided by default by Bazel. Replace calls to {@link
 * javax.tools.ToolProvider#getSystemJavaCompiler} with calls to {@link
 * BazelJavaCompiler#newInstance}.
 *
 * <p>This class is typically used only from a host build tool or in tests. When using this in
 * production, langtools.jar and the bootclasspath jars are deployed as separate jar files within
 * the runfiles directory.
 */
public class BazelJavaCompiler {

  // The default blessed javac options.

  private static final String DEFAULT_BOOTCLASSPATH = JavacBootclasspath.asString();

  private static final ImmutableList<String> DEFAULT_JAVACOPTS =
      ImmutableList.<String>builder()
          .addAll(JavaBuilderConfig.defaultJavacOpts())
          .add("-encoding")
          .add("UTF-8")
          // The bootclasspath must be specified both via an invocation option and
          // via fileManager.setLocation(PLATFORM_CLASS_PATH), to work around what
          // appears to be a bug in jdk[6,8] javac.
          .add("-bootclasspath")
          .add(DEFAULT_BOOTCLASSPATH)
          .build();

  private static final Class<? extends JavaCompiler> JAVA_COMPILER_CLASS = getJavaCompilerClass();

  /**
   * We use the platform classloader (on JDK 9) or bootstrap classloader (on JDK 8) as the parent
   * classloader instead of the default "system" class loader; we intentionally do not consult the
   * classpath. This way the class path is not polluted, we reduce the risk of having multiple
   * langtools on the classpath, and langtools.jar is only opened if this method is called. And this
   * will reduce problems for automated java dependency analysis, which other teams are trying to
   * do.
   */
  private static class LangtoolsClassLoader extends URLClassLoader {

    public LangtoolsClassLoader() throws MalformedURLException {
      super(new URL[] {getLangtoolsJar().toURI().toURL()}, getPlatformClassLoaderInternal());
    }

    private static ClassLoader getPlatformClassLoaderInternal() {
      try {
        return (ClassLoader) ClassLoader.class.getMethod("getPlatformClassLoader").invoke(null);
      } catch (ReflectiveOperationException e) {
        // Java 8
        return null;
      }
    }
  }

  private static Class<? extends JavaCompiler> getJavaCompilerClass() {
    try {
      ClassLoader cl = new LangtoolsClassLoader();
      return getJavaCompilerClass(cl);
    } catch (Exception e) {
      throw new RuntimeException("Cannot get java compiler", e);
    }
  }

  private static Class<? extends JavaCompiler> getJavaCompilerClass(ClassLoader cl)
      throws Exception {
    return Class.forName("com.sun.tools.javac.api.JavacTool", true, cl)
        .asSubclass(JavaCompiler.class);
  }

  /** Returns the langtools jar. */
  public static File getLangtoolsJar() {
    return JavaLangtools.file();
  }

  /** Returns the default javacopts, including the blessed bootclasspath. */
  public static List<String> getDefaultJavacopts() {
    return DEFAULT_JAVACOPTS;
  }

  /**
   * Returns a new {@link JavaCompiler} that has behavior as similar as possible to the java
   * compiler provided by default by the bazel build system, independent of the user-specified
   * {@code JAVABASE}.
   *
   * <p>More precisely, this method works analogously to {@link
   * javax.tools.ToolProvider#getSystemJavaCompiler}, but returns a {@code JavaCompiler} that
   * differs in these details:
   *
   * <ul>
   *   <li> uses the blessed javac implementation: {@code //tools/defaults:java_langtools}, as
   *       defined by bazel's --java_langtools flag.
   *   <li> uses the blessed boot class path: {@code //tools/defaults:javac_bootclasspath}, as
   *       defined by bazel's --javac_bootclasspath flag.
   *   <li> uses the blessed default values for javac options such as {@code -source}
   * </ul>
   *
   * <p>This class ensures that (by default) the {@code -source}, {@code -target} and {@code
   * -bootclasspath} flags all agree and specify the same (blessed) JDK version, for language and
   * API compatibility.
   *
   * <p>This method finds the javac implementation using a custom classloader that does not consult
   * the user's classpath. This works well, unless the return value is cast to a
   * javac-implementation class like {@code JavacTask}, in which case the dreaded classloader error
   * "can't cast JavacTaskImpl to JavacTask" raises its ugly head, in which case you should use
   * {@link #newInstance(ClassLoader)} instead.
   */
  public static JavaCompiler newInstance() {
    try {
      return newInstance(JAVA_COMPILER_CLASS.getConstructor().newInstance());
    } catch (Exception e) {
      throw new RuntimeException("Cannot get java compiler", e);
    }
  }

  /**
   * Returns a new {@link JavaCompiler} that has behavior as similar as possible to the java
   * compiler provided by default by bazel, independent of the user-specified {@code JAVABASE}.
   *
   * <p>This method has effect identical to {@link #newInstance()} (and that method is generally
   * preferred to this one), except that the javac implementation is found via the provided
   * classloader instead of defining a custom classloader that knows the standard location of the
   * blessed javac implementation.
   *
   * <p>This method is needed when the return value is cast to a javac-implementation class like
   * {@code JavacTask}, to avoid the dreaded multiple classloader error "can't cast JavacTaskImpl to
   * JavacTask".
   *
   * <p>Typically, users should pass {@code ClassLoader.getSystemClassLoader()} as the argument to
   * this method.
   */
  public static JavaCompiler newInstance(ClassLoader cl) {
    try {
      return newInstance(getJavaCompilerClass(cl).getConstructor().newInstance());
    } catch (Exception e) {
      throw new RuntimeException("Cannot get java compiler", e);
    }
  }

  private static JavaCompiler newInstance(final JavaCompiler delegate) {
    // We forward most operations to the JavaCompiler implementation in langtools.jar.
    return new JavaCompiler() {
      @Override
      public CompilationTask getTask(
          Writer out,
          JavaFileManager fileManager,
          DiagnosticListener<? super JavaFileObject> diagnosticListener,
          Iterable<String> options,
          Iterable<String> classes,
          Iterable<? extends JavaFileObject> compilationUnits) {
        // We prepend bazel's default javacopts to user javacopts,
        // so that the user can override them. javac supports this
        // "last option wins" style of option override.
        ImmutableList.Builder<String> fullOptions = ImmutableList.builder();
        fullOptions.addAll(getDefaultJavacopts());
        if (options != null) {
          fullOptions.addAll(options);
        }
        return delegate.getTask(
            out, fileManager, diagnosticListener, fullOptions.build(), classes, compilationUnits);
      }

      @Override
      public StandardJavaFileManager getStandardFileManager(
          DiagnosticListener<? super JavaFileObject> diagnosticListener,
          Locale locale,
          Charset charset) {
        StandardJavaFileManager fileManager =
            delegate.getStandardFileManager(diagnosticListener, locale, charset);

        try {
          fileManager.setLocation(
              StandardLocation.PLATFORM_CLASS_PATH, // bootclasspath
              JavacBootclasspath.asFiles());
        } catch (IOException e) {
          // Should never happen, according to javadocs for setLocation
          throw new RuntimeException(e);
        }
        return fileManager;
      }

      @Override
      public int run(InputStream in, OutputStream out, OutputStream err, String... arguments) {
        // prepend bazel's default javacopts to user arguments
        List<String> args =
            ImmutableList.<String>builder().addAll(getDefaultJavacopts()).add(arguments).build();
        return delegate.run(in, out, err, args.toArray(new String[0]));
      }

      @Override
      public Set<SourceVersion> getSourceVersions() {
        return delegate.getSourceVersions();
      }

      @Override
      public int isSupportedOption(String option) {
        return delegate.isSupportedOption(option);
      }
    };
  }
}