Java 将字符串动态生成字节码的实现方法

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;
  }
}

}