Javassist进行annoations问题

本文介绍了Javassist进行annoations问题的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我试图用了Javassist生成我的实体类。一切都很顺利，直到我加入了GeneratedValue注释ID字段。 @Id注释工作正常，但是当我加入@GeneeratedValue我得到一个异常。这是我的code：

I'm trying to generate my Entity class using javassist. Everything went well until I added the GeneratedValue annotation to the Id field. The @Id annotation works fine but when I add @GeneeratedValue I get an exception. This is my code:

ClassPool cp = ClassPool.getDefault(); CtClass ctClass = cp.makeClass("test.Snake"); ClassFile classFile = ctClass.getClassFile(); classFile.setVersionToJava5(); AnnotationsAttribute attribute = new AnnotationsAttribute(classFile.getConstPool(), AnnotationsAttribute.visibleTag); Annotation idAnnotation = new Annotation(classFile.getConstPool(), ClassPool.getDefault().get("javax.persistence.Id")); attribute.addAnnotation(idAnnotation); Annotation gvAnnotation = new Annotation(classFile.getConstPool(), ClassPool.getDefault().get("javax.persistence.GeneratedValue")); attribute.addAnnotation(gvAnnotation); CtField idField = new CtField(ClassPool.getDefault().get("java.lang.Long"), "id", ctClass); idField.getFieldInfo().addAttribute(attribute); ctClass.addField(idField); CtField nameField = new CtField(ClassPool.getDefault().get("java.lang.String"), "name", ctClass); ctClass.addField(nameField); AnnotationsAttribute attr = new AnnotationsAttribute(classFile.getConstPool(), AnnotationsAttribute.visibleTag); Annotation annotation = new Annotation(classFile.getConstPool(), ClassPool.getDefault().get("javax.persistence.Entity")); attr.addAnnotation(annotation); classFile.addAttribute(attr); snakeClass = ctClass.toClass(); newInstance = snakeClass.newInstance();

这是例外，我得到：

And this is the exception I get:

java.lang.NullPointerException at javassist.bytecode.ConstPool.getUtf8Info(ConstPool.java:565) at javassist.bytecode.annotation.EnumMemberValue.getValue(EnumMemberValue.java:98) at javassist.bytecode.annotation.EnumMemberValue.write(EnumMemberValue.java:116) at javassist.bytecode.annotation.Annotation.write(Annotation.java:316) at javassist.bytecode.AnnotationsAttribute.setAnnotations(AnnotationsAttribute.java:246) at javassist.bytecode.AnnotationsAttribute.addAnnotation(AnnotationsAttribute.java:211) at ClassLoadingTest.javassitTest(ClassLoadingTest.java:56)

这似乎是与@GeneratedValue一个问题。当我使用它独自ID内部消除我要么得到此异常。当我使用Eclipse调试器来观看变量的值，我得到得到这个

It seems to be a problem with @GeneratedValue. When I use it alone whithout id I get this exception either. When I use eclipse debugger to watch variable values, I get get this

com.sun.jdi.InvocationException occurred invoking method.

而不是注释值。但对于标识标注就说明了Javassist注释对象。

instead of the annotation value. but for Id annotation it shows a javassist annotation object.

我是新来了Javassist。谁能帮我？

I'm new to javassist. Can anyone help me?

推荐答案

我猜你不是在找什么发生了（我有同样的问题今天），但如果你这样做......

I guess you're not looking for what happened anymore (I had the same problem today), but if you do...

在使用构造注释（ConstPool CP，CtClass clazz所）Javassist是pre-会为注记类所有成员（见Annotation.java，第98行）。

When using constructor Annotation(ConstPool cp, CtClass clazz) javassist pre-creates all members for that Annotation Class (see Annotation.java, line 98).

在这种情况下，很容易因为有一个明确的评论：// TODO枚举不支持现在 （线101），正如你可以看到javax.persistence.GeneratedValue有一个叫类型GenerationType这是一个枚举的战略成员。

In this case it's easy because there is an explicit comment: "// todo Enums are not supported right now." (line 101) and as you can see in javax.persistence.GeneratedValue there is a member called strategy of type GenerationType which is an Enum.

不过，如果注解类有型类的任何成员，它不会工作，在子类的MemberValue.write方法引起一个NullPointerException异常。

Though if the Annotation class have any Members of type class it won't work, causing a NullPointerException on the MemberValue.write method of descendant classes.

解决方案或解决方法是你做了什么，用另一个构造留下手动添加成员，或（不认为这是一个不错的选择），设置在每个注释类成员的实例。

The solution or workaround is what you have done, using another constructor that leaves the members to be manually added, or (don't think this is a good option) set an instance for each class member in Annotation.

PS：我使用了Javassist 3.12.1.GA

PS: I'm using javassist 3.12.1.GA