The technical principles and practical experience sharing of the Swift Annotations framework in the Java class library
The technical principles and practical experience sharing of the Swift Annotations framework in the Java class library
Foreword
In Java development, we often use annotations to provide metadata for obtaining and processing at runtime.The Swift Annitations framework is a powerful annotation and generation tool that provides a simpler and more flexible way for Java developers to use annotations.This article will introduce the technical principles of the Swift Annotations framework and share some practical application experience.
1. Technical principles of Swift Annotions framework
The core concept of the Swift Annitations framework is based on the compile-time operation processor.It scan and process the annotations in the compilation stage through the annotation processor, and generate additional code.
1. Note definition
Swift Annitations framework allows developers to customize annotations.By using the Java meta annotation (Meta Annotations), we can define an annotation to indicate that this is an annotation of a Swift Annotations framework.For example:
@Retention(RetentionPolicy.SOURCE)
@Target(ElementType.TYPE)
public @interface SwiftAnnotation {
String value();
}
In the above examples, we used the retention strategy and use goals of the annotation to specify the annotation of the annotation.`` @RETENTION (RetentionPolicy.source) `It means that this annotation will only be retained in the source code and will not appear in the compiled file.`@Target (ElementType.type)` means that the annotation can be applied to classes statement.
2. Comment processor
In order to use the Swift Annotions framework, we need to create a custom annotation processor.The annotation processor is the operating environment of the Java compiler when processing the annotation.It scan the annotation in the source code and perform the corresponding processing logic.
@SupportedAnnotationTypes("com.example.SwiftAnnotation")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class SwiftAnnotationProcessor extends AbstractProcessor {
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
// Processing Swift Annotations Note Logic
return true;
}
}
In the above example, we used the annotation of the annotations that need to be processed.`@Supportedsourceversion` Note used to specify the Java version of supported support.In the `PROCESS" method, we can write specific annotation processing logic.
3. Note processing logic
The annotation processor can process annotations in several ways, such as generating additional Java class, modifying existing Java classes, and writing the annotation information into configuration files.The following is an example, showing how to generate a new Java class:
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
for (Element element : roundEnv.getElementsAnnotatedWith(SwiftAnnotation.class)) {
// Get the injection value
SwiftAnnotation annotation = element.getAnnotation(SwiftAnnotation.class);
String value = annotation.value();
// Generate a new Java class
String newClassName = value + "SwiftGenerated";
String newClassContent = "public class " + newClassName + " {}";
try {
JavaFileObject sourceFile = processingEnv.getFiler()
.createSourceFile(newClassName, element);
try (Writer writer = sourceFile.openWriter()) {
writer.write(newClassContent);
}
} catch (IOException e) {
e.printStackTrace();
}
}
return true;
}
In the above example, we obtain elements that are marked by the annotation of the `Swiftannotation` method by` GetelementsannotatedWith`.Then, we obtain the value of the annotation through the `Getannotation" method.Finally, we use the `CreateSourceFile" method to generate a new Java source file and write the new class content into the file.
Second, the practical experience sharing of Swift Annotions framework
When the actual application of the Swift Annitations framework, we can follow some of the following practical experience:
1. Reasonable selection of the retention strategy of the annotation
Select the appropriate annotation retention strategy according to actual needs (`@Retention`).If the annotation is no longer needed after compilation, you can choose `RetentionPolicy.source` to retain the strategy.If the annotation needs to be used at runtime, you can choose the retaining strategy of `RetentionPolicy.runtime`.
2. Annotation Objective Selection
Select the right target according to the use scenario of the annotation (`@target`).For example, if the annotation can only be applied to the method, you can choose `Elementtype.Method`.
3. Understand the life cycle of the annotation processor
The annotation processor can be executed during multiple compilation stages, and it is important to understand the processing logic and restrictions at different stages.The annotation processor has methods such as `Init`,` Process`, and `End`, which can be processed in turn.
4. Carefully deal with the circular dependence of annotations
When multiple annotation processors have a cycle dependence, they may cause dead cycle during compilation.When dealing with annotations, you should consider avoiding cycle dependence or solving this problem through other ways.
Summarize
The Swift Annitations framework is a powerful annotation and generation tool that can simplify the use of annotations in Java development.This article introduces the technical principles of the Swift Annotions framework and share some practical experience.Through reasonable definition annotations and processors, we can use and manage annotations more efficiently to improve code quality and development efficiency.
Reference materials:
-SWIFT ANNOTINS official documentation
