blocks|key|2796760|text|在运行批注处理器时，您不能访问已编译的类。批注处理的要点是它是预编译的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2796761|相反，您需要创建一个专门处理您的注释类型的注释处理器，然后使用镜像API访问该字段。例如：|2796762|@SupportedAnnotationTypes("com.example.MyAnnotation")
public+class+CompileTimeAnnotationProcessor+extends+AbstractProcessor+{

++++@Override
++++public+boolean+process(Set<?+extends+TypeElement>+annotations,+
+++++++++++++++++++++++++++RoundEnvironment+roundEnv)+{
++++++++//+Only+one+annotation,+so+just+use+annotations.iterator().next();
++++++++Set<?+extends+Element>+elements+=+roundEnv.getElementsAnnotatedWith(
++++++++++++++++annotations.iterator().next());
++++++++Set<VariableElement>+fields+=+ElementFilter.fieldsIn(elements);
++++++++for+(VariableElement+field+:+fields)+{
++++++++++++TypeMirror+fieldType+=+field.asType();
++++++++++++String+fullTypeClassName+=+fieldType.toString();
++++++++++++//+Validate+fullTypeClassName
++++++++}
++++++++return+true;
++++}
}|code-block|syntax|javascript|2796763|对于验证，您不能使用像MyType.class这样的东西来使用任何尚未编译的类(包括那些即将使用注释编译的类)。对于这些，您必须仅使用的字符串。这是因为批注处理发生在称为“源代码生成”的预编译阶段，这允许您在编译器使用批注运行之前生成源代码。|offset|length|style|CODE|BOLD|2796764|验证字段类型是否为java.lang.String+(已编译)的示例验证：|2796765|for+(VariableElement+field+:+fields)+{
++++TypeMirror+fieldType+=+field.asType();
++++String+fullTypeClassName+=+fieldType.toString();
++++if+(!String.class.getName().equals(fullTypeClassName))+{
++++++++processingEnv.getMessager().printMessage(
++++++++++++++++Kind.ERROR,+"Field+type+must+be+java.lang.String",+field);
++++}
}|2796766|资源|2796767|2796768|Main+APT+Page|unordered-list-item|2796769|Mirror+API+Javadocs+(Java+7+and+older)|2796770|Edit:+Mirror+API+Javadocs+(Java+8)+|2796771|请注意，镜像应用程序接口现在已在javax.lang.model下的Java8中标准化，旧的应用程序接口已弃用。有关详细信息，请参阅this+blog+post。如果您一直在使用javax类，那么就不需要使用worry.|2796772|2796773|2796774|​|2796775|编辑：|2796776|2796777|我想要获取字段类型，以获得该类型的注释。但这似乎不太可能？|blockquote|2796778|2796779|2796780|这确实是可能的！这可以在TypeMirror上使用更多的方法来完成|2796781|if+(fieldType.getKind()+!=+TypeKind.DECLARED)+{
++++processingEnv.getMessager().printMessage(
++++++++++++Kind.ERROR,+"Field+cannot+be+a+generic+type.",+field);
}
DeclaredType+declaredFieldType+=+(DeclaredType)+fieldType;
TypeElement+fieldTypeElement+=+(TypeElement)+declaredFieldType.asElement();|2796782|从这里开始，您有两个选择：|2796783|2796784|如果您试图查找的批注已经编译(即，它来自另一个库)，那么您可以直接引用类来获取annotation.|ordered-list-item|2796785|If您试图查找的批注没有编译(即，它是在运行APT的javac的当前调用中编译的)，那么您可以通过AnnotationMirror实例引用它。|2796786|2796787|已编译|2796788|DifferentAnnotation+diffAnn+=+fieldTypeElement.getAnnotation(
++++++++DifferentAnnotation.class);
//+Process+diffAnn|2796789|非常简单，这使您可以直接访问注释本身。|2796790|未编译|2796791|请注意，无论注释是否被编译，这个解决方案都可以工作，只是它不像上面的代码那样干净。|2796792|下面是我曾经写过的几个方法，它们通过类名称从注释镜像中提取特定的值：|2796793|private+static+<T>+T+findAnnotationValue(Element+element,+String+annotationClass,
++++++++String+valueName,+Class<T>+expectedType)+{
++++T+ret+=+null;
++++for+(AnnotationMirror+annotationMirror+:+element.getAnnotationMirrors())+{
++++++++DeclaredType+annotationType+=+annotationMirror.getAnnotationType();
++++++++TypeElement+annotationElement+=+(TypeElement)+annotationType
++++++++++++++++.asElement();
++++++++if+(annotationElement.getQualifiedName().contentEquals(
++++++++++++++++annotationClass))+{
++++++++++++ret+=+extractValue(annotationMirror,+valueName,+expectedType);
++++++++++++break;
++++++++}
++++}
++++return+ret;
}

private+static+<T>+T+extractValue(AnnotationMirror+annotationMirror,
++++++++String+valueName,+Class<T>+expectedType)+{
++++Map<ExecutableElement,+AnnotationValue>+elementValues+=+new+HashMap<ExecutableElement,+AnnotationValue>(
++++++++++++annotationMirror.getElementValues());
++++for+(Entry<ExecutableElement,+AnnotationValue>+entry+:+elementValues
++++++++++++.entrySet())+{
++++++++if+(entry.getKey().getSimpleName().contentEquals(valueName))+{
++++++++++++Object+value+=+entry.getValue().getValue();
++++++++++++return+expectedType.cast(value);
++++++++}
++++}
++++return+null;
}|2796794|假设您正在寻找DifferentAnnotation注释，源代码如下所示：|2796795|@DifferentAnnotation(name+=+"My+Class")
public+class+MyClass+{

++++@MyAnnotation
++++private+String+field;

++++//+...
}|2796796|此代码将打印My+Class|2796797|String+diffAnnotationName+=+findAnnotationValue(fieldTypeElement,
++++++++"com.example.DifferentAnnotation",+"name",+String.class);
System.out.println(diffAnnotationName);|2796798|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/7/docs/technotes/guides/apt/|1|http://docs.oracle.com/javase/7/docs/jdk/api/apt/mirror/overview-summary.html|2|https://docs.oracle.com/javase/8/docs/api/javax/lang/model/package-summary.html|3|https://blogs.oracle.com/darcy/entry/an_apt_replacement^0|0|0|0|B|C|1W|1H|0|0|11|9|G|0|0|0|2|0|0|0|D|0|D|0|0|0|12|0|12|1|0|0|5|6|S|6|S|2|1|0|32|G|G|2H|5|1U|E|3|1|0|0|0|1|0|0|3|0|0|0|T|0|0|0|0|X|C|A|0|0|0|D|0|0|0|1E|0|0|1Z|Q|5|1D|G|0|0|0|3|0|0|0|J|0|0|3|0|0|15|0|0|Y|0|0|0|11|7|J|0|0|0|E|6|8|0|0^^$0|@$1|2|3|4|5|6|7|2U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|2V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|2W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|2X|8|@$K|2Y|L|2Z|M|N]|$K|30|L|31|M|O]]|9|@]|A|$]]|$1|P|3|Q|5|6|7|32|8|@$K|33|L|34|M|O]|$K|35|L|36|M|N]]|9|@]|A|$]]|$1|R|3|S|5|F|7|37|8|@]|9|@]|A|$G|H]]|$1|T|3|U|5|6|7|38|8|@$K|39|L|3A|M|O]]|9|@]|A|$]]|$1|V|3|-4|5|6|7|3B|8|@]|9|@]|A|$]]|$1|W|3|X|5|Y|7|3C|8|@$K|3D|L|3E|M|O]]|9|@$K|3F|L|3G|1|3H]]|A|$]]|$1|Z|3|10|5|Y|7|3I|8|@$K|3J|L|3K|M|O]]|9|@$K|3L|L|3M|1|3N]]|A|$]]|$1|11|3|12|5|Y|7|3O|8|@$K|3P|L|3Q|M|O]|$K|3R|L|3S|M|O]]|9|@$K|3T|L|3U|1|3V]]|A|$]]|$1|13|3|14|5|Y|7|3W|8|@$K|3X|L|3Y|M|O]|$K|3Z|L|40|M|N]|$K|41|L|42|M|N]]|9|@$K|43|L|44|1|45]]|A|$]]|$1|15|3|-4|5|6|7|46|8|@]|9|@]|A|$]]|$1|16|3|-4|5|6|7|47|8|@]|9|@]|A|$]]|$1|17|3|18|5|6|7|48|8|@$K|49|L|4A|M|O]]|9|@]|A|$]]|$1|19|3|1A|5|6|7|4B|8|@$K|4C|L|4D|M|O]]|9|@]|A|$]]|$1|1B|3|-4|5|6|7|4E|8|@]|9|@]|A|$]]|$1|1C|3|1D|5|1E|7|4F|8|@$K|4G|L|4H|M|O]]|9|@]|A|$]]|$1|1F|3|-4|5|6|7|4I|8|@]|9|@]|A|$]]|$1|1G|3|-4|5|6|7|4J|8|@]|9|@]|A|$]]|$1|1H|3|1I|5|6|7|4K|8|@$K|4L|L|4M|M|O]|$K|4N|L|4O|M|N]]|9|@]|A|$]]|$1|1J|3|1K|5|F|7|4P|8|@]|9|@]|A|$G|H]]|$1|1L|3|1M|5|6|7|4Q|8|@$K|4R|L|4S|M|O]]|9|@]|A|$]]|$1|1N|3|-4|5|6|7|4T|8|@]|9|@]|A|$]]|$1|1O|3|1P|5|1Q|7|4U|8|@$K|4V|L|4W|M|O]]|9|@]|A|$]]|$1|1R|3|1S|5|1Q|7|4X|8|@$K|4Y|L|4Z|M|O]|$K|50|L|51|M|N]|$K|52|L|53|M|N]]|9|@]|A|$]]|$1|1T|3|-4|5|6|7|54|8|@]|9|@]|A|$]]|$1|1U|3|1V|5|6|7|55|8|@$K|56|L|57|M|O]]|9|@]|A|$]]|$1|1W|3|1X|5|F|7|58|8|@]|9|@]|A|$G|H]]|$1|1Y|3|1Z|5|6|7|59|8|@$K|5A|L|5B|M|O]]|9|@]|A|$]]|$1|20|3|21|5|6|7|5C|8|@$K|5D|L|5E|M|O]]|9|@]|A|$]]|$1|22|3|23|5|6|7|5F|8|@$K|5G|L|5H|M|O]]|9|@]|A|$]]|$1|24|3|25|5|6|7|5I|8|@$K|5J|L|5K|M|O]]|9|@]|A|$]]|$1|26|3|27|5|F|7|5L|8|@]|9|@]|A|$G|H]]|$1|28|3|29|5|6|7|5M|8|@$K|5N|L|5O|M|O]|$K|5P|L|5Q|M|N]]|9|@]|A|$]]|$1|2A|3|2B|5|F|7|5R|8|@]|9|@]|A|$G|H]]|$1|2C|3|2D|5|6|7|5S|8|@$K|5T|L|5U|M|O]|$K|5V|L|5W|M|N]]|9|@]|A|$]]|$1|2E|3|2F|5|F|7|5X|8|@]|9|@]|A|$G|H]]|$1|2G|3|-4|5|6|7|5Y|8|@]|9|@]|A|$]]]|2H|$2I|$5|2J|2K|2L|A|$2M|2N]]|2O|$5|2J|2K|2L|A|$2M|2P]]|2Q|$5|2J|2K|2L|A|$2M|2R]]|2S|$5|2J|2K|2L|A|$2M|2T]]]]

When running your annotation processor, you don't have access to the compiled classes. The point of annotation processing is that it happens pre-compile.

Instead, you need to create an annotation processor that specifically handles your annotation type, then use the mirror API to access the field. For example:

<pre><code>@SupportedAnnotationTypes("com.example.MyAnnotation")
public class CompileTimeAnnotationProcessor extends AbstractProcessor {

 @Override
 public boolean process(Set&lt;? extends TypeElement&gt; annotations, 
 RoundEnvironment roundEnv) {
 // Only one annotation, so just use annotations.iterator().next();
 Set&lt;? extends Element&gt; elements = roundEnv.getElementsAnnotatedWith(
 annotations.iterator().next());
 Set&lt;VariableElement&gt; fields = ElementFilter.fieldsIn(elements);
 for (VariableElement field : fields) {
 TypeMirror fieldType = field.asType();
 String fullTypeClassName = fieldType.toString();
 // Validate fullTypeClassName
 }
 return true;
 }
}
</code></pre>

For the validation, you cannot use any classes which have yet to be compiled (including those that are about to be compiled with the annotation) using something like <code>MyType.class</code>. For these, you must use strings only. That is because annotation processing occurs during a pre-compiling phase known as "source generation", which is what allows you to generate source code before the compiler runs using annotations.

An example validation verifying that the field type is <code>java.lang.String</code> (which is already compiled):

<pre><code>for (VariableElement field : fields) {
 TypeMirror fieldType = field.asType();
 String fullTypeClassName = fieldType.toString();
 if (!String.class.getName().equals(fullTypeClassName)) {
 processingEnv.getMessager().printMessage(
 Kind.ERROR, "Field type must be java.lang.String", field);
 }
}
</code></pre>

Resources

<ul>
<li><a href="http://docs.oracle.com/javase/7/docs/technotes/guides/apt/" rel="noreferrer">Main APT Page</a></li>
<li><a href="http://docs.oracle.com/javase/7/docs/jdk/api/apt/mirror/overview-summary.html" rel="noreferrer">Mirror API Javadocs (Java 7 and older)</a></li>
<li>Edit: <a href="https://docs.oracle.com/javase/8/docs/api/javax/lang/model/package-summary.html" rel="noreferrer">Mirror API Javadocs (Java 8)</a>

<ul>
<li>Note that the mirror API is now standardized in Java 8 under <code>javax.lang.model</code> and the old API is deprecated. See <a href="https://blogs.oracle.com/darcy/entry/an_apt_replacement" rel="noreferrer">this blog post</a> for more information. If you've been using the <code>javax</code> classes, then you don't need to worry.</li>
</ul></li>
</ul>

Edit:

<blockquote>
 I want to get the field type to get annotations on that type. But this does not seem like it will be possible?
</blockquote>

Indeed it is possible! This can be done using more methods on the <code>TypeMirror</code>:

<pre><code>if (fieldType.getKind() != TypeKind.DECLARED) {
 processingEnv.getMessager().printMessage(
 Kind.ERROR, "Field cannot be a generic type.", field);
}
DeclaredType declaredFieldType = (DeclaredType) fieldType;
TypeElement fieldTypeElement = (TypeElement) declaredFieldType.asElement();
</code></pre>

From here, you have two choices:

<ol>
<li>If the annotation you're trying to find is already compiled (i.e. it's from another library) then you can reference the class directly to get the annotation.</li>
<li>If the annotation you're trying to find is not compiled (i.e. it's being compiled in the current call to <code>javac</code> that's running the APT) then you can reference it via <code>AnnotationMirror</code> instances.</li>
</ol>

Already Compiled

<pre><code>DifferentAnnotation diffAnn = fieldTypeElement.getAnnotation(
 DifferentAnnotation.class);
// Process diffAnn
</code></pre>

Very straight-forward, this gives you direct access to the annotation itself.

Not Compiled

Note that this solution will work regardless of whether or not the annotation is compiled, it's just not as clean as the code above.

Here are a couple methods I wrote once to extract a certain value from an annotation mirror by its class name:

<pre><code>private static &lt;T&gt; T findAnnotationValue(Element element, String annotationClass,
 String valueName, Class&lt;T&gt; expectedType) {
 T ret = null;
 for (AnnotationMirror annotationMirror : element.getAnnotationMirrors()) {
 DeclaredType annotationType = annotationMirror.getAnnotationType();
 TypeElement annotationElement = (TypeElement) annotationType
 .asElement();
 if (annotationElement.getQualifiedName().contentEquals(
 annotationClass)) {
 ret = extractValue(annotationMirror, valueName, expectedType);
 break;
 }
 }
 return ret;
}

private static &lt;T&gt; T extractValue(AnnotationMirror annotationMirror,
 String valueName, Class&lt;T&gt; expectedType) {
 Map&lt;ExecutableElement, AnnotationValue&gt; elementValues = new HashMap&lt;ExecutableElement, AnnotationValue&gt;(
 annotationMirror.getElementValues());
 for (Entry&lt;ExecutableElement, AnnotationValue&gt; entry : elementValues
 .entrySet()) {
 if (entry.getKey().getSimpleName().contentEquals(valueName)) {
 Object value = entry.getValue().getValue();
 return expectedType.cast(value);
 }
 }
 return null;
}
</code></pre>

Let's say that you're looking for the <code>DifferentAnnotation</code> annotation and your source code looks like this:

<pre><code>@DifferentAnnotation(name = "My Class")
public class MyClass {

 @MyAnnotation
 private String field;

 // ...
}
</code></pre>

This code will print <code>My Class</code>:

<pre><code>String diffAnnotationName = findAnnotationValue(fieldTypeElement,
 "com.example.DifferentAnnotation", "name", String.class);
System.out.println(diffAnnotationName);
</code></pre>

I am writing my first Annotations processor and having trouble with something that seems trivial but I cannot find any information about it.

I have a element annotated with my annotation

<pre><code>@MyAnnotation String property;
</code></pre>

When I get this property as a element in my processor I can not seem to get the type of the element in any way. In this case a would want to get a Class or TypeElement instance representing String. 

I tried instantiating a class object of the container type with <code>Class.forName()</code> but it threw a ClassNotFoundException. I think this is because I do not have access to the class loader containing the class?

Get field class in annotations processor

编译器

容器

我正在编写我的第一个注解处理器，在一些看似微不足道的事情上遇到了麻烦，但我找不到任何有关它的信息。我用我的注解注释了一个元素@MyAnnotation String property;当我在处理器中将此属性作为元素获取时，似乎无法以任何方式获取元素的类型。在这种情况下，需要获取表示字符串的类或TypeElement实例...

问在批注处理器中获取字段类
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问在批注处理器中获取字段类EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问在批注处理器中获取字段类
EN