APT处理annotation

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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）

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

[java] view plaincopy
@Documented  
@Target(ElementType.TYPE)  
@Retention(RetentionPolicy.SOURCE)  
public @interface Persistent  
{  
    String table();  
}  

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

以上是三个Annotation类型

[java] view plaincopy
@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

[java] view plaincopy
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();  
                    }  
                }  
            }  
        }  
    }  
}  

[java] view plaincopy
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

[xhtml] view plaincopy
<?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>  

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