链接绑定 Linked Bindings
Guice中最常用的一种绑定为Linked Bindings，或者起名为链接绑定。
链接绑定是指把一个类型映射到它的一个具体实现上。下面这个例子就是把接口（TransactionLog ）映射到它的一个实现上（DatabaseTransactionLog）：
Linked bindings map a type to its implementation. This example maps the interface TransactionLog to the implementation DatabaseTransactionLog:
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public class BillingModule extends AbstractModule { @Override protected void configure() { bind(TransactionLog.class).to(DatabaseTransactionLog.class); } } |
当你执行 injector.getInstance(TransactionLog.class)调用，或者injector遇到依赖于TransactionLog时，那么程序将实际使用DatabaseTransactionLog。可以将类型“链接”（映射）到它的任意子类型上，比如链接到它的一个实现类或一个扩展类上。你甚至还可以把具体的DatabaseTransactionLog 类（不是接口）链接到它的子类上。
Now, when you call injector.getInstance(TransactionLog.class), or when the injector encounters a dependency on TransactionLog, it will use a DatabaseTransactionLog. Link from a type to any of its subtypes, such as an implementing class or an extending class. You can even link the concrete DatabaseTransactionLog class to a subclass:
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bind(DatabaseTransactionLog.class).to(MySqlDatabaseTransactionLog.class); |
这种链接绑定可以是“链式”串联的：
Linked bindings can also be chained:
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public class BillingModule extends AbstractModule { @Override protected void configure() { bind(TransactionLog.class).to(DatabaseTransactionLog.class); bind(DatabaseTransactionLog.class).to(MySqlDatabaseTransactionLog.class); } } |
在这种情况下，当需要TransactionLog类型的值时，注入器提供的是MySqlDatabaseTransactionLog类型的值。
In this case, when a TransactionLog is requested, the injector will return a MySqlDatabaseTransactionLog.
实例
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public interface HelloService { void sayHello(); } public class HelloServiceImpl implements HelloService { @Override public void sayHello() { System.out.println("hello"); } } |
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import com.google.inject.AbstractModule; public class HelloServiceModule extends AbstractModule { @Override protected void configure() { bind(HelloService.class).to(HelloServiceImpl.class); // bind(HelloServiceImpl.class).to(SubHelloServiceImpl.class); } } public class Test { public static void main(String[] args) { Injector injector = Guice.createInjector(new HelloServiceModule()); HelloService helloService = injector.getInstance(HelloService.class); System.out.println(helloService.getClass().getSimpleName());//打印源代码中给出的‘底层类’简称 helloService.sayHello(); } } |
执行结果：
HelloServiceImpl
Hello
还可以写成链式绑定，此时再添加个子类：
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public class SubHelloServiceImpl extends HelloServiceImpl { @Override public void sayHello() { System.out.println("Sub Hello"); } } |
然后将之前HelloServiceModule里的注释去掉。
执行结果：
SubHelloServiceImpl
Sub Hello
实例参考来源：http://lifestack.cn/archives/85.html
下一节：绑定注释 Binding Annotations
说明：
鉴于网上guice中文资料较少，出于个人爱好，对该项目下的用户API文档进行翻译。如有翻译不恰当之处，还望指正。
google Guice 项目地址：https://github.com/google/guice
Guice 英文API地址：https://github.com/google/guice/wiki/LinkedBindings
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