尽管警告要我放弃目前的做法，但我目前看不到更好的方法来解决我的问题。我必须 生成 在运行时运行 Java 代码，然后编译它、加载它并引用它。

问题是生成的代码导入了系统类加载器已经加载的代码(我想)——也就是说，代码存在于我的类路径中的一个 jar 中。 (我在 Java 6 上的 Tomcat 6 Web 容器中运行。)您可能会问自己为什么这是个问题 - 好吧我肯定不知道 - 但事实是我遇到了编译错误:

/W:/.../parser/v0.5/AssignELParser.java:6: package com.xxx.yyy.zzz.configuration does not exist

根据互联网上的一些示例，我定义了以下类:

class MemoryClassLoader extends ChainedAction {

    private static final Logger LOG = Logger.getLogger(MemoryClassLoader.class);

    private LoaderImpl impl;

    private class LoaderImpl extends ClassLoader {

        // The compiler tool
        private final JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

        // Compiler options
        private final Iterable<String> options = Arrays.asList("-verbose");

        // DiagnosticCollector, for collecting compilation problems
        private final DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();

        // Our FileManager
        private final MemoryFileManager manager = new MemoryFileManager(this.compiler);

        public LoaderImpl(File sourceDirectory) {

            List<Source> list = new ArrayList<Source>();

            File[] files = sourceDirectory.listFiles(new FilenameFilter() {

                @Override
                public boolean accept(File dir, String name) {

                    return name.endsWith(Kind.SOURCE.extension);
                }
            });

            for (File file : files) {
                list.add(new Source(file));
            }

            CompilationTask task = compiler.getTask(null, manager, diagnostics, options, null, list);
            Boolean compilationSuccessful = task.call();

            LOG.info("Compilation has " + ((compilationSuccessful) ? "concluded successfully" : "failed"));

            // report on all errors to screen
            for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics.getDiagnostics()) {
                LOG.warn(diagnostic.getMessage(null));
            }
        }

        @Override
        protected Class<?> findClass(String name) throws ClassNotFoundException {
            synchronized (this.manager) {
                Output output = manager.map.remove(name);
                if (output != null) {
                    byte[] array = output.toByteArray();
                    return defineClass(name, array, 0, array.length);
                }
            }
            return super.findClass(name);
        }
    }

    @Override
    protected void run() {  

        impl = new LoaderImpl(new File(/* Some directory path */));

    }
}



class MemoryFileManager extends ForwardingJavaFileManager<JavaFileManager> {

    final Map<String, Output> map = new HashMap<String, Output>();

    MemoryFileManager(JavaCompiler compiler) {
        super(compiler.getStandardFileManager(null, null, null));
    }

    @Override
    public Output getJavaFileForOutput(Location location, String name, Kind kind, FileObject source) {

        Output output = new Output(name, kind);
        map.put(name, output);

        return output;
    }

}


class Output extends SimpleJavaFileObject {

    private final ByteArrayOutputStream baos = new ByteArrayOutputStream();

    Output(String name, Kind kind) {
        super(URI.create("memo:///" + name.replace('.', '/') + kind.extension), kind);
    }

    byte[] toByteArray() {
        return this.baos.toByteArray();
    }

    @Override
    public ByteArrayOutputStream openOutputStream() {
        return this.baos;
    }
}



class Source extends SimpleJavaFileObject {


    public Source(File file) {
        super(file.toURI(), Kind.SOURCE);
    }


    @Override
    public CharSequence getCharContent(boolean ignoreEncodingErrors) {

        StringBuilder sb = new StringBuilder("");
        try {
            File file = new File(uri);
            FileReader fr = new FileReader(file);
            BufferedReader br = new BufferedReader(fr);

            sb = new StringBuilder((int) file.length());
            String line = "";
            while ((line = br.readLine()) != null) {
                sb.append(line);
                sb.append("\n");
            }
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return sb.toString();
    }
}

似乎通过扩展 ClassLoader 类而不调用显式 super 构造函数的内部类 LoaderImpl 应该引用系统类加载器作为其父类加载器。

如果是这样，那为什么我会收到上面的“运行时”编译错误？为什么找不到导入类的代码？

最佳答案

不确定它是否有帮助，但您是否尝试过明确指定类路径？

getClassPath()
{
  ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
  URL[] urls = ((URLClassLoader) classLoader).getURLs();
  StringBuilder buf = new StringBuilder(1000);
  buf.append(".");
  String separator = System.getProperty("path.separator");
  for (URL url : urls) {
      buf.append(separator).append(url.getFile());
  }
}

classPath = buf.toString();

然后

options.add("-classpath");
options.add(getClassPath());

我也看不到你在哪里将 LoaderImpl 实例传递给 compiler。不应该明确地完成吗？

关于java类加载器和运行时编译，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/1642606/