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：
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：
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
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