Java 將字符串動態生成字節碼的實現方法

可以生成可執行的class文件

直接上能執行代碼：
代碼如下：

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("內部錯誤。");
   }
  } else {
   // / new CalculationException("錯誤的表達式。");
  }
  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;
  }
}

}