Java注解处理器
2018-09-03 本文已影响12人
编码前线
概述
注解处理器（Annotation Processor），是
javac的一个工具，用来在编译时扫描和处理注解。一个注解处理器以Java代码（或者编译过得字节码）作为输入，生成
.java文件作为输出。
接下来我们模仿ButterKnife 实现一个@BindView的注解来了解Java注解处理器的使用。
创建项目
apt-project.png
模块说明：
bindview-annotation：定义注解，@BindView。
bindview-compiler：定义注解处理器，处理被@BindView标记的代码，并在编译时生成xxxActivity_ViewBinding.java
bindview-api：工具类，调用xxxActivity_ViewBinding.java中的方法，实现View的绑定。
bindview-annotation(自定义注解)
创建注解类BindView
@Retention(RetentionPolicy.CLASS) // 表示编译时注解
@Target(ElementType.FIELD) // 表示注解范围为类成员
public @interface BindView {
int value(); // 用于获取对应View的id
}
bindview-compiler(注解处理器)
在该module的build.gradle中添加如下代码：
dependencies {
implementation 'com.google.auto.service:auto-service:1.0-rc3'
implementation project(':bindview-annotation')
}
创建BindViewProcessor
// 该注解用来自动生成META-INF/services/javax.annotation.processing.Processor文件，
// 并在该文件注册BindViewProcessor这个注解处理器
@AutoService(Processor.class)
public class BindViewProcessor extends AbstractProcessor {
// 用来在处理注解的过程中打印日志
private Messager mMessager;
private Elements mElementUtils;
private Map<String, ClassCreatorProxy> mProxyMap = new HashMap<>();
@Override
public synchronized void init(ProcessingEnvironment processingEnvironment) {
super.init(processingEnvironment);
mMessager = processingEnvironment.getMessager();
mElementUtils = processingEnvironment.getElementUtils();
}
/**
* 声明该注解所处理的注解类型，也可以直接在注解处理器的类声明添加如下注解：
* @SupportedAnnotationTypes({"com.github.xch168.bindview.annotation.BindView"})
* @return 接受处理的所有注解类型的集合
*/
@Override
public Set<String> getSupportedAnnotationTypes() {
return Collections.singleton(BindView.class.getCanonicalName());
}
/**
* 指定使用的Java版本
* @return
*/
@Override
public SourceVersion getSupportedSourceVersion() {
return SourceVersion.latestSupported();
}
@Override
public boolean process(Set<? extends TypeElement> set, RoundEnvironment roundEnvironment) {
mMessager.printMessage(Diagnostic.Kind.NOTE, "processing...");
mProxyMap.clear();
// 得到所有的注解
Set<? extends Element> elements = roundEnvironment.getElementsAnnotatedWith(BindView.class);
for (Element element : elements) {
VariableElement variableElement = (VariableElement) element;
TypeElement classElement = (TypeElement) variableElement.getEnclosingElement();
String fullClassName = classElement.getQualifiedName().toString();
ClassCreatorProxy proxy = mProxyMap.get(fullClassName);
if (proxy == null) {
proxy = new ClassCreatorProxy(mElementUtils, classElement);
mProxyMap.put(fullClassName, proxy);
}
BindView bindAnnotation = variableElement.getAnnotation(BindView.class);
int id = bindAnnotation.value();
proxy.putElement(id, variableElement);
}
// 通过遍历mProxy，创建java文件
for (String key : mProxyMap.keySet()) {
ClassCreatorProxy proxyInfo = mProxyMap.get(key);
mMessager.printMessage(Diagnostic.Kind.NOTE, " --> create " + proxyInfo.getProxyClassFullName());
try {
JavaFileObject jfo = processingEnv.getFiler().createSourceFile(proxyInfo.getProxyClassFullName(), proxyInfo.getTypeElement());
Writer writer = jfo.openWriter();
writer.write(proxyInfo.generateJavaCode());
writer.flush();
writer.close();
} catch (IOException e) {
e.printStackTrace();
mMessager.printMessage(Diagnostic.Kind.NOTE, " --> create " + proxyInfo.getProxyClassFullName() + " error!");
}
}
mMessager.printMessage(Diagnostic.Kind.NOTE, "process finish ...");
return true;
}
}
ClassCreatorProxy是创建Java代码的代理类：
public class ClassCreatorProxy {
private String mBindingClassName;
private String mPackageName;
private TypeElement mTypeElement;
private Map<Integer, VariableElement> mVariableElementMap = new HashMap<>();
public ClassCreatorProxy(Elements elementUtils, TypeElement classElement) {
mTypeElement = classElement;
PackageElement packageElement = elementUtils.getPackageOf(mTypeElement);
String packageName = packageElement.getQualifiedName().toString();
String className = mTypeElement.getSimpleName().toString();
mPackageName = packageName;
mBindingClassName = className + "_ViewBinding";
}
public void putElement(int id, VariableElement element) {
mVariableElementMap.put(id, element);
}
public String generateJavaCode() {
StringBuilder builder = new StringBuilder();
builder.append("package ").append(mPackageName).append(";\n\n");
builder.append("\n");
builder.append("public class ").append(mBindingClassName);
builder.append(" {\n");
generateMethods(builder);
builder.append("\n");
builder.append("}\n");
return builder.toString();
}
private void generateMethods(StringBuilder builder) {
builder.append("public void bind(" + mTypeElement.getQualifiedName() + " host) {\n");
for (int id : mVariableElementMap.keySet()) {
VariableElement element = mVariableElementMap.get(id);
String name = element.getSimpleName().toString();
String type = element.asType().toString();
builder.append("host." + name).append(" = ");
builder.append("(" + type + ")(((android.app.Activity)host).findViewById(" + id + "));\n");
}
builder.append(" }\n");
}
public String getProxyClassFullName() {
return mPackageName + "." + mBindingClassName;
}
public TypeElement getTypeElement() {
return mTypeElement;
}
}
bindview-api(注解生成代码的调用工具类)
创建注解工具类BindViewTool
public class BindViewTool {
/**
* 通过翻车找到对应的ViewBinding类，然后调用其中的bind方法，完成View的绑定
* @param activity
*/
public static void bind(Activity activity) {
Class clz = activity.getClass();
try {
Class bindViewClass = Class.forName(clz.getName() + "_ViewBinding");
Method method = bindViewClass.getMethod("bind", activity.getClass());
method.invoke(bindViewClass.newInstance(), activity);
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
}
使用注解
在app模块的build.gradle中添加如下代码：
dependencies {
implementation project(':bindview-annotation')
implementation project(':bindview-api')
// Gradle 2.2及后面的版本都是用annotationProcessor，早先的版本是用apt，现以废弃，故不再介绍
annotationProcessor project(':bindview-compiler')
}
在MainActivity中使用注解：
public class MainActivity extends AppCompatActivity {
@BindView(R.id.content)
TextView mContentText;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
BindViewTool.bind(this);
mContentText.setText("From BindView");
}
}
运行后生成的代码（路径 app/build/generated/source/apt）：
apt-generated-code.png
MainActivity_ViewBinding代码：
public class MainActivity_ViewBinding {
public void bind(com.github.xch168.annotationdemo.MainActivity host) {
host.mContentText = (android.widget.TextView)(((android.app.Activity)host).findViewById(2131165228));
}
}
通过javapoet生成代码
上面生成代码的部分，是通过字符串拼接，过程非常繁琐。接下来就介绍一种更优雅的方式，使用javapoet。
添加依赖：
dependencies {
implementation 'com.squareup:javapoet:1.10.0'
}
在ClassCreatorProxy中添加如下代码
public class ClassCreatorProxy {
// ……
public TypeSpec generateJavaCodeByJavapoet() {
TypeSpec bindingClass = TypeSpec.classBuilder(mBindingClassName)
.addModifiers(Modifier.PUBLIC)
.addMethod(generateMethodsByJavapoet())
.build();
return bindingClass;
}
private MethodSpec generateMethodsByJavapoet() {
ClassName host = ClassName.bestGuess(mTypeElement.getQualifiedName().toString());
MethodSpec.Builder methodBuilder = MethodSpec.methodBuilder("bind")
.addModifiers(Modifier.PUBLIC)
.returns(void.class)
.addParameter(host, "host");
for (int id : mVariableElementMap.keySet()) {
VariableElement element = mVariableElementMap.get(id);
String name = element.getSimpleName().toString();
String type = element.asType().toString();
methodBuilder.addCode("host." + name + " = " + "(" + type + ")(((android.app.Activity)host).findViewById(" + id + "));");
}
return methodBuilder.build();
}
public String getPackageName() {
return mPackageName;
}
}
在BindViewProcessor中调用：
@Override
public boolean process(Set<? extends TypeElement> set, RoundEnvironment roundEnvironment) {
// ...
for (String key : mProxyMap.keySet()) {
ClassCreatorProxy proxyInfo = mProxyMap.get(key);
JavaFile javaFile = JavaFile.builder(proxyInfo.getPackageName(), proxyInfo.generateJavaCodeByJavapoet()).build();
try {
// 生成文件
javaFile.writeTo(processingEnv.getFiler());
} catch (IOException e) {
e.printStackTrace();
}
}
mMessager.printMessage(Diagnostic.Kind.NOTE, "process finish ...");
}
相比用StringBuilder拼Java代码，明显简介很多，且生成的代码是一样的。
编码前线.jpg
