疯狂JAVA讲义---第十四章：APT处理annotation

APT是一种java提供的注释处理工具，他能对annotation做额外的处理。

我们就用hibernate的annotation来做例子，eg：（代码如下，我就不多说了）

@Documented @Target(ElementType.FIELD) @Retention(RetentionPolicy.SOURCE) public @interface IdProperty { String column(); String type(); String generator(); }

@Documented @Target(ElementType.TYPE) @Retention(RetentionPolicy.SOURCE) public @interface Persistent { String table(); }

@Documented @Target(ElementType.FIELD) @Retention(RetentionPolicy.SOURCE) public @interface Property { String column(); String type(); }

以上是三个Annotation类型

@Persistent(table="persons_table") public class Person { @IdProperty(column="person_id",type="integer",generator="identity") private int id; @Property(column="person_name",type="string") private String name; @Property(column="person_age",type="integer") private int age; public Person() { } public Person(int id , String name , int age) { this.id = id; this.name = name; this.age = age; } public void setId(int id) { this.id = id; } public int getId() { return this.id; } public void setName(String name) { this.name = name; } public String getName() { return this.name; } public void setAge(int age) { this.age = age; } public int getAge() { return this.age; } }

以上是要处理的类

public class HibernateAnnotationProcessor implements AnnotationProcessor { //Annotation处理器环境，是该处理器与APT交互的重要途径 private AnnotationProcessorEnvironment env; //构造HibernateAnnotationProcessor对象时，获得处理器环境 public HibernateAnnotationProcessor(AnnotationProcessorEnvironment env) { this.env = env; } //循环处理每个对象 public void process() { //遍历每个class文件 for (TypeDeclaration t : env.getSpecifiedTypeDeclarations()) { //定义一个文件输出流，用于生成额外的文件 FileOutputStream fos = null; //获取正在处理的类名 String clazzName = t.getSimpleName(); //获取类定义前的Persistent Annotation Persistent per = t.getAnnotation(Persistent.class); //当per Annotation不为空时才继续处理 if(per != null) { try { //创建文件输出流 fos = new FileOutputStream(clazzName + ".hbm.xml"); PrintStream ps = new PrintStream(fos); //执行输出 ps.println("<?xml version=/"1.0/"?>"); ps.println("<!DOCTYPE hibernate-mapping"); ps.println(" PUBLIC /"-//Hibernate/Hibernate Mapping DTD 3.0//EN/""); ps.println(" /"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd/">"); ps.println("<hibernate-mapping>"); ps.print(" <class name=/"" + t); //输出per的table()的值 ps.println("/" table=/"" + per.table() + "/">"); for (FieldDeclaration f : t.getFields()) { //获取指定FieldDeclaration前面的IdProperty Annotation IdProperty id = f.getAnnotation(IdProperty.class); //如果id Annotation不为空 if (id != null) { //执行输出 ps.println(" <id name=/"" + f.getSimpleName() + "/" column=/"" + id.column() + "/" type=/"" + id.type() + "/">"); ps.println(" <generator class=/"" + id.generator() + "/"/>"); ps.println(" </id>"); } //获取指定FieldDeclaration前面的Property Annotation Property p = f.getAnnotation(Property.class); //如果p Annotation不为空 if (p != null) { //执行输出 ps.println(" <property name=/"" + f.getSimpleName() + "/" column=/"" + p.column() + "/" type=/"" + p.type() + "/"/>"); } } ps.println(" </class>"); ps.println("</hibernate-mapping>"); } catch (Exception e) { e.printStackTrace(); } finally { //关闭输出流 try { if (fos != null) { fos.close(); } } catch (IOException ex) { ex.printStackTrace(); } } } } } }

public class HibernateAnnotationFactory implements AnnotationProcessorFactory { //所有支持的注释类型 public Collection<String> supportedAnnotationTypes() { return Arrays.asList("Property" , "IdProperty" , "Persistent"); } //返回所有支持的选项 public Collection<String> supportedOptions() { return Arrays.asList(new String[0]); } //返回Annotation处理器 public AnnotationProcessor getProcessorFor(Set<AnnotationTypeDeclaration> atds, AnnotationProcessorEnvironment env) { return new HibernateAnnotationProcessor(env); } }

处理机和工厂

最后用apt命令：apt -factory HibernateAnnotationFactory Person.java

这样就生成了hibernate的.hbm.xml文件

<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name="Person" table="persons_table"> <id name="id" column="person_id" type="integer"> <generator class="identity"/> </id> <property name="name" column="person_name" type="string"/> <property name="age" column="person_age" type="integer"/> </class> </hibernate-mapping>