Java 将字符串动态生成字节码的一种方法

可以生成可执行的class文件

 直接上能执行代码：

public class Test {

 /**

  * @param args

  */

 @SuppressWarnings("static-access")

 public static void main(String[] args) {

  try {

   new Test().calculate("234 - ( 1 + 45 * 4 ) / 5");

  } catch (Exception e) {

   e.printStackTrace();

  }

 }

 private static double calculate(String expr) throws Exception {

  String className = "CalculatorMain";

  String methodName = "calculate";

  String source = "class " + className

    + " { public static void main(String[] args){System.out.println(calculate());} public static double " + methodName + "() { return "

    + expr + "; } }";

  JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

  StandardJavaFileManager fileManager = compiler.getStandardFileManager(

    null, null, null);

  StringSourceJavaObject sourceObject = new Test.StringSourceJavaObject(

    "Main", source);

  Iterable<? extends JavaFileObject> fileObjects = Arrays

    .asList(sourceObject);

  CompilationTask task = compiler.getTask(null, fileManager, null, null,

    null, fileObjects);

  boolean result = task.call();

  if (result) {

   ClassLoader loader = Thread.currentThread().getClass()

     .getClassLoader();

   try {

    Class<?> clazz = loader.loadClass(className);

    Method method = clazz.getMethod(methodName, new Class<?>[] {});

    Object value = method.invoke(null, new Object[] {});

    return (Double)value;

   } catch (Exception e) {

    // throw new CalculationException("内部错误。");

   }

  } else {

   // / new CalculationException("错误的表达式。");

  }

  return 0;

 }

 static class StringSourceJavaObject extends SimpleJavaFileObject {

  private String content = null;

  public StringSourceJavaObject(String name, String content)

    throws URISyntaxException {

   super(URI.create("string:///" + name.replace('.', '/')

     + Kind.SOURCE.extension), Kind.SOURCE);

   this.content = content;

  }

  public CharSequence getCharContent(boolean ignoreEncodingErrors)

    throws IOException {

   return content;

  }

 }

}