如何将JDK6 ToolProvider和JavaCompiler与上下文类加载器一起使用？

问题描述

我的用例是使用JDK 6中提供的ToolProvider和JavaCompiler类从java程序编译生成的源文件。源文件包含对上下文类加载器中的类的引用（它在J2EE容器中运行），但不包含在系统类加载器。我的理解是，默认情况下，ToolProvider将使用系统类加载器创建JavaCompiler实例。

My usage case is compiling generated source files from a java program using the ToolProvider and JavaCompiler classes provided in JDK 6. The source files contain references to classes in the context classloader (it runs in a J2EE container), but not in the system classloader. My understanding is that by default the ToolProvider will create the JavaCompiler instance with the system classloader.

有没有办法为JavaCompiler指定要使用的类加载器？

Is there a way to specify a classloader for JavaCompiler to use?

我尝试过这种方法，在IBM DeveloperWorks上修改过：

I tried this approach, modified from something on IBM DeveloperWorks:

FileManagerImpl fm =
    new FileManagerImpl(compiler.getStandardFileManager(null, null, null););

将FileManagerImpl定义为：

with FileManagerImpl defined as:

static final class FileManagerImpl
    extends ForwardingJavaFileManager<JavaFileManager> {

   public FileManagerImpl(JavaFileManager fileManager) {
      super(fileManager);
   }

   @Override
   public ClassLoader getClassLoader(JavaFileManager.Location location) {
      new Exception().printStackTrace();
      return Thread.currentThread().getContextClassLoader();
   }

}

堆栈跟踪表明它只被调用一次在注释处理期间。我验证了要编译的源文件中引用的类不在系统类路径中，但可以从上下文类加载器中获得。

The stacktrace indicates it's only called once during annotation processing. I verified the class referenced in the source file to be compiled is not on the system classpath but is available from the context classloader.

推荐答案

如果您知道contextclassloader已知的文件的类路径，则可以将它们传递给编译器：

If you know the classpath to the files that are known to the contextclassloader you can pass them to the compiler:

    StandardJavaFileManager fileManager = compiler.getStandardFileManager(this /* diagnosticlistener */, null, null);
// get compilationunits from somewhere, for instance via fileManager.getJavaFileObjectsFromFiles(List<file> files)
List<String> options = new ArrayList<String>();
options.add("-classpath");
StringBuilder sb = new StringBuilder();
URLClassLoader urlClassLoader = (URLClassLoader) Thread.currentThread().getContextClassLoader();
for (URL url : urlClassLoader.getURLs())
    sb.append(url.getFile()).append(File.pathSeparator);
options.add(sb.toString());
CompilationTask task = compiler.getTask(null, fileManager, this /* diagnosticlistener */, options, null, compilationUnits);
task.call();

此示例假设您使用的是URLClassloader（允许您检索类路径），但您可以如果你愿意，可以插入自己的类路径。

This example assumes you're using a URLClassloader (which allows you to retrieve the classpath) but you could insert your own classpath if you wanted to.

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