JAVA CDI 学习(4) - @Alternative/@Default/@Any & Extension

时间：2021-09-21 22:53:02

前面几节学习到的CDI内容，基本上都是hard-code，以硬编码的方式在代码里指定注入类型，这并非依赖注入的本意，依赖注入的优势之一在于“解耦”，这一节我们将学习如何利用配置来动态注入的类型及属性初始化。

一、@Alternative/@Default/@Any

当一个服务接口（也称契约）有多个实现时，可以在代码里指定一个缺省的实现类型（即：标注成@Default或@Any），其它实现类标注成@Alternative，以后如果需要动态切换实现类，只要在webapp/WEB-INF/beans.xml中配置即可。

1.1 新建二个示例接口

 package contract;

 public interface Connection {

     String connect();

 }

Connection

该接口模拟db连接，里面有一个connect方法，用来连接db.

 package contract;

 public interface DriveService {

     String drive();

 }

DriveService

该接口模拟游戏应用中，有些人物具有驾驶技能。

1.2 提供接口实现

假设Connection有二个实现，一个用来连接到Oracle Database，另一个用来连接Microsoft Sql Server

 package contract.impl;

 import javax.enterprise.inject.Default;

 import contract.Connection;

 @Default

 public class OracleConnection implements Connection {

     @Override

     public String connect() {

         return "Oracle Database is connecting...";

     }

 }

OracleConnection

 package contract.impl;

 import javax.enterprise.inject.Alternative;

 import contract.Connection;

 @Alternative

 public class SqlServerConnection implements Connection {

     @Override

     public String connect() {

         return "Microsoft SqlServer is connecting...";

     }

 }

SqlServerConnection

注：OracleConnection上应用了注解@Default，表示这是接口Connection的默认实现类（@Default实质上是系统的默认注解，其实也可以省略，系统会自动默认为@Default）;SqlServerConnection上应用了注解@Alternative，表示它是候选项，俗称：备胎：），所有非@Default的实现类，都必须标识@Alternative，否则注入时，会提示“不明确的类型”

再来看DriveService的实现，我们提供三种实现：驾驶汽车、摩托车、拖拉机

 package contract.impl;

 import contract.DriveService;

 public class CarDriveImpl implements DriveService {

     @Override

     public String drive() {

         String msg = "Drive a car...";

         System.out.println(msg);

         return msg;

     }

 }

CarDriveImpl

 package contract.impl;

 import javax.enterprise.inject.Alternative;

 import contract.DriveService;

 @Alternative

 public class MotorcycleDriveImpl implements DriveService {

     @Override

     public String drive() {

         String msg = "Drive a motocycle...";

         System.out.println(msg);

         return msg;

     }

 }

MotorcycleDriveImpl

 package contract.impl;

 import javax.enterprise.inject.Alternative;

 import contract.DriveService;

 @Alternative

 public class TractorDriveImpl implements DriveService {

     @Override

     public String drive() {

         String msg = "Drive a tractor...";

         System.out.println(msg);

         return msg;

     }

 }

TractorDriveImpl

注：MotocycleDriveImpl、TractorDriveImpl这二个类使用了@Alternative，即它们俩是候选，剩下的CarDriveImpl上未使用任何注解，即默认的@Default

1.3 编写Controller类

 package controller;

 import javax.inject.Inject;

 import javax.inject.Named;

 import contract.Connection;

 @Named("Conn")

 public class ConnectionController {

     @Inject

     private Connection conn;

     public Connection getConn() {

         return conn;

     }

 }

ConnectionController

 package controller;

 import javax.enterprise.inject.*;

 import javax.inject.Inject;

 import javax.inject.Named;

 import contract.DriveService;

 @Named("Drive")

 public class DriveController {

     @Inject

     private DriveService driveService;

     public DriveService getDriveService() {

         return driveService;

     }

     @Inject

     @Any

     private Instance<DriveService> anySerInstance;

     public DriveService getAnySerInstance() {

         return anySerInstance.get();

     }

 }

DriveController

注：DriveController中anySerInstance成员上使用了@Any，从本例最终使用的效果上看，它跟@Default一样，只不过细节要留意一下，需要使用Instance<T>接口，这点跟@Default有点不同。

1.4 UI层

 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

 <html xmlns="http://www.w3.org/1999/xhtml"

       xmlns:h="http://java.sun.com/jsf/html"

       xmlns:f="http://java.sun.com/jsf/core"

       xmlns:ui="http://java.sun.com/jsf/facelets"> 

 <h:head>

     <title>CDI - Alternative/Default/Any</title>

 </h:head>

 <body>

     #{Drive.driveService.drive()}

     <br />

     <br />#{Drive.anySerInstance.drive()}

     <br />

     <br /> #{Conn.conn.connect()}

 </body>

 </html>

Index.xhtml

运行结果：

JAVA CDI 学习(4) - @Alternative/@Default/@Any & Extension

修改beans.xml的内容如下：

 <?xml version="1.0" encoding="UTF-8"?>

 <beans xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

     xsi:schemaLocation="         http://java.sun.com/xml/ns/javaee          http://java.sun.com/xml/ns/javaee/beans_1_0.xsd">

     <alternatives>

         <class>contract.impl.SqlServerConnection</class>

         <class>contract.impl.TractorDriveImpl</class>

     </alternatives>

 </beans>

beans.xml

重新在Jboss里部署、运行，结果如下：

JAVA CDI 学习(4) - @Alternative/@Default/@Any & Extension

在不修改java源代码的前提下，仅通过配置文件beans.xml的修改，就动态切换了接口的实现类。

二、Extension

不仅注入的类型可以由配置文件来动态切换，也可以由配置文件来直接初始化注入对象的属性值（虽然我个人认为这种场景在实际开发中其实并不多见）

2.1 先来定义几个类：

BaseDto.java

 package dto;

 import java.io.Serializable;

 public class BaseDto implements Serializable {

     private static final long serialVersionUID = 804047416541420712L;

     public BaseDto() {

         System.out.println("BaseDto's constructor is called...");

     }

 }

BaseDto

 package dto;

 @DtoType(ProductType.Product)

 public class Product extends BaseDto {

     public Product() {

         System.out.println("Product's constructor is called...");

     }

     private static final long serialVersionUID = 7364741422914624828L;

     private String productNo;

     private String productName;

     public String getProductName() {

         return productName;

     }

     public void setProductName(String productName) {

         this.productName = productName;

     }

     public String getProductNo() {

         return productNo;

     }

     public void setProductNo(String productNo) {

         this.productNo = productNo;

     }

     @Override

     public String toString() {

         return "productNo：" + productNo + " , productName: " + productName

                 + " , serialVersionUID:" + serialVersionUID;

     }

 }

Product

 package dto;

 //@DtoType(ProductType.Computer)

 public class Computer extends Product {

     public Computer() {

         System.out.println("Computer's constructor is called...");

     }

     private static final long serialVersionUID = -5323881568748028893L;

     private String cpuType;

     private int hardDiskCapacity;

     public String getCpuType() {

         return cpuType;

     }

     public void setCpuType(String cpuType) {

         this.cpuType = cpuType;

     }

     public int getHardDiskCapacity() {

         return hardDiskCapacity;

     }

     public void setHardDiskCapacity(int hardDiskCapacity) {

         this.hardDiskCapacity = hardDiskCapacity;

     }

     @Override

     public String toString() {

         return "productNo：" + getProductNo() + " , productName: "

                 + getProductName() + " , cpuType:" + getCpuType()

                 + " , hardDiskCapacity: " + getHardDiskCapacity()

                 + " , serialVersionUID:" + serialVersionUID;

     }

 }

Computer

 package dto;

 //@DtoType(ProductType.Cloth)

 public class Cloth extends Product {

     private static final long serialVersionUID = -8799705022666106476L;

     private String brand;

     public String getBrand() {

         return brand;

     }

     public void setBrand(String brand) {

         this.brand = brand;

     }

     @Override

     public String toString() {

         return "productNo：" + getProductNo() + " , productName: "

                 + getProductName() + " , brand:" + getBrand()

                 + " , serialVersionUID:" + serialVersionUID;

     }

 }

Cloth

Product上使用了一个自定义的注解：@DtoType

 package dto;

 import javax.inject.Qualifier;

 import java.lang.annotation.Retention;

 import java.lang.annotation.RetentionPolicy;

 @Qualifier

 @Retention(RetentionPolicy.RUNTIME)

 public @interface DtoType {

     public ProductType value();

 }

@DtoType

以及枚举：

 package dto;

 public enum ProductType {

     Product,Computer,Cloth

 }

ProductType

2.2 BaseDtoExtension

为了实现注入配置化，我们还需要对BaseDto写一个扩展类：

 package dto.extension;

 import java.io.IOException;

 import java.io.InputStream;

 import java.util.logging.Logger;

 import javax.enterprise.event.Observes;

 import javax.enterprise.inject.spi.*;

 import javax.xml.parsers.*;

 import dto.*;

 import org.w3c.dom.*;

 import org.xml.sax.SAXException;

 public class BaseDtoExtension implements Extension {

     private final Document document;

     private final Logger log = Logger.getLogger(BaseDtoExtension.class

             .getName());

     public BaseDtoExtension() {

         try {

             InputStream creatureDefs = BaseDtoExtension.class.getClassLoader()

                     .getResourceAsStream("inject-beans.xml");

             DocumentBuilderFactory factory = DocumentBuilderFactory

                     .newInstance();

             DocumentBuilder builder = factory.newDocumentBuilder();

             document = builder.parse(creatureDefs);

         } catch (ParserConfigurationException e) {

             throw new RuntimeException("Error building xml parser, aborting", e);

         } catch (SAXException e) {

             throw new RuntimeException("SAX exception while parsing xml file",

                     e);

         } catch (IOException e) {

             throw new RuntimeException("Error reading or parsing xml file", e);

         }

     }

     <X extends BaseDto> void processInjectionTarget(

             @Observes ProcessInjectionTarget<X> pit) {

         Class<? extends BaseDto> klass = pit.getAnnotatedType().getJavaClass();

         log.info("Setting up injection target for " + klass);

         final Element entry = (Element) document.getElementsByTagName(

                 klass.getSimpleName().toLowerCase()).item(0);

         pit.setInjectionTarget(new XmlWrappedInjection<X>(pit

                 .getInjectionTarget(), entry));

     }

 }

BaseDtoExtension

该扩展，将读取resources/inject-beans.xml文件的内容，并完成BaseDto以及所有子类的加载，包括Inject，该类还使用了另一个辅助类：

 package dto.extension;

 import java.lang.reflect.Field;

 import java.util.Set;

 import javax.enterprise.context.spi.CreationalContext;

 import javax.enterprise.inject.InjectionException;

 import javax.enterprise.inject.spi.*;

 import dto.*;

 import org.w3c.dom.Element;

 public class XmlWrappedInjection<X extends BaseDto> implements

         InjectionTarget<X> {

     private final InjectionTarget<X> wrapped;

     private final Element xmlBacking;

     public XmlWrappedInjection(InjectionTarget<X> it, Element xmlElement) {

         wrapped = it;

         xmlBacking = xmlElement;

     }

     @Override

     public void inject(X instance, CreationalContext<X> ctx) {

         wrapped.inject(instance, ctx);

         final Class<? extends BaseDto> klass = instance.getClass();

         //yjm注：出于演示目的，这里仅反射了本类中声明的field，所以注入时，父类中的field会被忽略，大家可以自行修改，逐层向上反射，直到BaseDto类为止

         for (Field field : klass.getDeclaredFields()) {

             field.setAccessible(true);

             final String fieldValueFromXml = xmlBacking.getAttribute(field

                     .getName());

             try {

                 //System.out.println("the filed name is :" + field.getName());

                 if (field.getName().toLowerCase().equals("serialversionuid")) {

                     continue;

                 }

                 //注：出于演示目的，这里只处理了int、long、String这三种类型，其它类型大家可自行扩展

                 if (field.getType().isAssignableFrom(Integer.TYPE)) {

                     field.set(instance, Integer.parseInt(fieldValueFromXml));

                 } else if (field.getType().isAssignableFrom(Long.TYPE)) {

                     field.set(instance, Long.parseLong(fieldValueFromXml));

                 } else if (field.getType().isAssignableFrom(String.class)) {

                     field.set(instance, fieldValueFromXml);

                 } else {

                     throw new InjectionException("Cannot convert to type "

                             + field.getType());

                 }

             } catch (IllegalAccessException e) {

                 throw new InjectionException("Cannot access field " + field);

             }

         }

     }

     @Override

     public void postConstruct(X instance) {

         wrapped.postConstruct(instance);

     }

     @Override

     public void preDestroy(X instance) {

         wrapped.preDestroy(instance);

     }

     @Override

     public X produce(CreationalContext<X> ctx) {

         return wrapped.produce(ctx);

     }

     @Override

     public void dispose(X instance) {

         wrapped.dispose(instance);

     }

     @Override

     public Set<InjectionPoint> getInjectionPoints() {

         return wrapped.getInjectionPoints();

     }

 }

XmlWrappedInjection

注：这里仅只是演示，所以处理得相对比较简单，如果一个类继承自父类，Inject时，上面的代码，只反射了子类本身声明的field，对于父类的属性，未逐层向上反射，大家可以自行改进。

2.3 控制器

 package controller;

 import javax.inject.Inject;

 import javax.inject.Named;

 import dto.Cloth;

 import dto.Computer;

 import dto.DtoType;

 import dto.Product;

 import dto.ProductType;

 @Named("Ext")

 public class ExtensionController {

     @Inject

     //@DtoType(ProductType.Computer)

     private Computer computer;

     @Inject

     //@DtoType(ProductType.Cloth)

     private Cloth cloth;

     @Inject

     @DtoType(ProductType.Product)

     private Product product;

     public Computer getComputer() {

         return computer;

     }

     public Cloth getCloth() {

         return cloth;

     }

     public Product getProduct() {

         return product;

     }

 }

ExtensionController

注：这里思考一下，为什么Product上必须使用注解@DtoType(ProductType.Product)，而其它二个Inject的field不需要？如果暂时没想明白的朋友，建议回到第一节 ，看下1.7节的内容，因为Computer、Cloth都继承自Product类，所以在实例Product类时，系统有3个选择：Computer、Cloth、Product，它不知道该选哪一个？所以运行时，系统会罢工，so，需要额外的注释给它一点提示。

2.4 ext.xhtml

 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

 <html xmlns="http://www.w3.org/1999/xhtml"

     xmlns:h="http://java.sun.com/jsf/html"

     xmlns:f="http://java.sun.com/jsf/core"

     xmlns:ui="http://java.sun.com/jsf/facelets">

 <h:head>

     <title>Extension Test</title>

 </h:head>

 <body>

     #{Ext.product.toString()}

     <br /> #{Ext.computer.toString()}

     <br /> #{Ext.cloth.toString()}

 </body>

 </html>

ext.xhtml

2.5 inject-beans.xml

 <?xml version="1.0" encoding="UTF-8"?>

 <basedtos>

     <product productNo="001" productName="A Unknown New Product" />

     <computer productNo="T60" productName="ThinkPad" cpuType="2-cores"

         hardDiskCapacity="320" />

     <cloth productNo="XX" productName="Underware" brand="JackJohns" />

 </basedtos>

inject-beans.xml

该文件设计时，要放在main/java/resources/目录下，部署时，会自动复制到webapp/resources/

2.6 javax.enterprise.inject.spi.Extension

/main/java/resources/META-INF/services目录下，新建一个文件：javax.enterprise.inject.spi.Extension，内容如下：

dto.extension.BaseDtoExtension

该文件的作用是在运行时，告诉系统根据BaseDtoExtension类的定义去找inject-beans.xml，它相当于入口。

2.7 运行效果：浏览地址 http://localhost:8080/cdi-alternative-sample/ext.jsf

JAVA CDI 学习(4) - @Alternative/@Default/@Any & Extension

跟预期结果完全一样，不过正如文中指出的一样，父类的属性被忽略了，如果父类成员也需要初始化，需要大家自行修改XmlWrappedInjection类

最后附示例源代码：cdi-alternative-sample.zip

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