Spring框架的依赖注入是家喻户晓的，但是在实际的开发中我们想使用便捷的依赖注入功能，但是又不想引入Spring框架的笨重和复杂性，该怎么办呢？

有了Google Guice，这个问题便简单了，首先在你的maven项目里引入

    
         
     
      com.google.inject
         
     
      guice
         
     
      4.0
     
    

官方文档里给出的例子又臭又长，我不使用官方的例子，下面我们来写个最简单的HelloWorld

import javax.inject.Singleton;import com.google.inject.Guice;import com.google.inject.Inject;import com.google.inject.Injector;@Singletonclass HelloPrinter {	public void print() {		System.out.println("Hello, World");	}}@Singletonpublic class Sample {	@Inject	private HelloPrinter printer;	public void hello() {		printer.print();	}	public static void main(String[] args) {		Injector injector = Guice.createInjector();		Sample sample = injector.getInstance(Sample.class);		sample.hello();	}}

我们使用Guice创建了一个注射器Injector，然后从Injector拿到你想要的对象就可以了，Guice会自动装配依赖树。Guice的启动速度是很快的，在一个大型应用中，Guice装配所有的模块决不会超过1s。Guice是一个非常干净的依赖注入框架，框架除了依赖注入功能之外，没有任何其它非相关模块功能。

Guice里最常用的两个注解就是@Singleton和@Inject，Singleton表示构建的对象是单例的，Inject表示被标注的字段将使用Guice自动注入。在一般的项目中这两个注解一般可以完成90%以上的装配工作。

Guice需要实例化对象，请确保相应被实例化的对象有默认构造器。

当某个接口有多个实现时，我们使用@ImplementedBy注解在接口定义上，指定接口的具体实现类

import javax.inject.Singleton;import com.google.inject.Guice;import com.google.inject.ImplementedBy;import com.google.inject.Inject;import com.google.inject.Injector;@ImplementedBy(SimpleHelloPrinter.class)interface IHelloPrinter {	void print();}@Singletonclass SimpleHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Simple World");	}}@Singletonclass ComplexHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Complex World");	}}@Singletonpublic class Sample {	@Inject	private IHelloPrinter printer;	public void hello() {		printer.print();	}	public static void main(String[] args) {		Injector injector = Guice.createInjector();		Sample sample = injector.getInstance(Sample.class);		sample.hello();	}}

如果我们不用Singleton标注，每次获取实例时，Guice会重新构造一个，这个会有反射构造器的性能损耗，在高性能场景下，请谨慎。

import com.google.inject.Guice;import com.google.inject.Inject;import com.google.inject.Injector;class HelloPrinter {	private static int counter;	private int myCounter;	public HelloPrinter() {		myCounter = counter++;	}	public void print() {		System.out.printf("Hello, World %d\n", myCounter);	}}public class Sample {	@Inject	private HelloPrinter printer;	public void hello() {		printer.print();	}	public static void main(String[] args) {		Injector injector = Guice.createInjector();		Sample sample = injector.getInstance(Sample.class);		sample.hello();		sample = injector.getInstance(Sample.class);		sample.hello();		sample = injector.getInstance(Sample.class);		sample.hello();		sample = injector.getInstance(Sample.class);		sample.hello();	}}

我们可以不使用@ImplementedBy注解，因为这样不优雅，谁会在定义接口的时候就能预知实现类的名称呢。我们可以使用Guice Module定义装配规则，它相当于Spring的XML文件，只不过它的装配规则都是使用代码定义的。你可能会辩解说代码定义怎么能比得上XML定义呢，其实Guice Module在一个大型项目中也是非常的简洁，一般只会占用几十行代码，Module里面配置的仅仅是特殊的专配规则。就规则的可读性而言，代码要比XML舒服的多。

import javax.inject.Singleton;import com.google.inject.AbstractModule;import com.google.inject.Guice;import com.google.inject.Inject;import com.google.inject.Injector;interface IHelloPrinter {	void print();}@Singletonclass SimpleHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Simple World");	}}@Singletonclass ComplexHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Complex World");	}}class SampleModule extends AbstractModule {	@Override	protected void configure() {		bind(IHelloPrinter.class).to(SimpleHelloPrinter.class);	}}@Singletonpublic class Sample {	@Inject	private IHelloPrinter printer;	public void hello() {		printer.print();	}	public static void main(String[] args) {		Injector injector = Guice.createInjector(new SampleModule());		Sample sample = injector.getInstance(Sample.class);		sample.hello();	}}

我们还可以使用@Named名称指令来指定依赖注入实现

import javax.inject.Named;import javax.inject.Singleton;import com.google.inject.AbstractModule;import com.google.inject.Guice;import com.google.inject.Inject;import com.google.inject.Injector;import com.google.inject.name.Names;interface IHelloPrinter {	void print();}@Singletonclass SimpleHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Simple World");	}}@Singletonclass ComplexHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Complex World");	}}class SampleModule extends AbstractModule {	@Override	protected void configure() {		bind(IHelloPrinter.class).annotatedWith(Names.named("simple")).to(SimpleHelloPrinter.class);		bind(IHelloPrinter.class).annotatedWith(Names.named("complex")).to(ComplexHelloPrinter.class);	}}@Singletonpublic class Sample {	@Inject	@Named("simple")	private IHelloPrinter simplePrinter;	@Inject	@Named("complex")	private IHelloPrinter complexPrinter;	public void hello() {		simplePrinter.print();		complexPrinter.print();	}	public static void main(String[] args) {		Injector injector = Guice.createInjector(new SampleModule());		Sample sample = injector.getInstance(Sample.class);		sample.hello();	}}

我们不使用字段注入，改用构造器注入，如果我们需要在构造器里做一些特别的初始化工作

import javax.inject.Named;import javax.inject.Singleton;import com.google.inject.AbstractModule;import com.google.inject.Guice;import com.google.inject.Inject;import com.google.inject.Injector;import com.google.inject.name.Names;interface IHelloPrinter {	void print();}@Singletonclass SimpleHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Simple World");	}}@Singletonclass ComplexHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Complex World");	}}class SampleModule extends AbstractModule {	@Override	protected void configure() {		bind(IHelloPrinter.class).annotatedWith(Names.named("simple")).to(SimpleHelloPrinter.class);		bind(IHelloPrinter.class).annotatedWith(Names.named("complex")).to(ComplexHelloPrinter.class);	}}@Singletonpublic class Sample {	@Named("simple")	private IHelloPrinter simplePrinter;	@Named("complex")	private IHelloPrinter complexPrinter;	@Inject	public Sample(@Named("simple") IHelloPrinter simplePrinter, @Named("complex") IHelloPrinter complexPrinter) {		this.simplePrinter = simplePrinter;		this.complexPrinter = complexPrinter;	}	public void hello() {		simplePrinter.print();		complexPrinter.print();	}	public static void main(String[] args) {		Injector injector = Guice.createInjector(new SampleModule());		Sample sample = injector.getInstance(Sample.class);		sample.hello();	}}

还可以自动注入Set，Map容器，但是得首先加上扩展库

    
         
     
      com.google.inject.extensions
         
     
      guice-multibindings
         
     
      4.0
     
    

注入Set

import java.util.Set;import javax.inject.Inject;import javax.inject.Singleton;import com.google.inject.AbstractModule;import com.google.inject.Guice;import com.google.inject.Injector;import com.google.inject.multibindings.Multibinder;interface IHelloPrinter {	void print();}@Singletonclass SimpleHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Simple World");	}}@Singletonclass ComplexHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Complex World");	}}class SampleModule extends AbstractModule {	@Override	protected void configure() {		Multibinder
    
      printers = Multibinder.newSetBinder(binder(), IHelloPrinter.class);		printers.addBinding().to(SimpleHelloPrinter.class);		printers.addBinding().to(ComplexHelloPrinter.class);	}}@Singletonpublic class Sample {	@Inject	private Set
     
       printers;	public void hello() {		for (IHelloPrinter printer : printers) {			printer.print();		}	}	public static void main(String[] args) {		Injector injector = Guice.createInjector(new SampleModule());		Sample sample = injector.getInstance(Sample.class);		sample.hello();	}}
     
    

注入Map

import java.util.Map;import javax.inject.Inject;import javax.inject.Singleton;import com.google.inject.AbstractModule;import com.google.inject.Guice;import com.google.inject.Injector;import com.google.inject.multibindings.MapBinder;interface IHelloPrinter {	void print();}@Singletonclass SimpleHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Simple World");	}}@Singletonclass ComplexHelloPrinter implements IHelloPrinter {	public void print() {		System.out.println("Hello, Complex World");	}}class SampleModule extends AbstractModule {	@Override	protected void configure() {		MapBinder
    
      printers = MapBinder.newMapBinder(binder(), String.class, IHelloPrinter.class);		printers.addBinding("simple").to(SimpleHelloPrinter.class);		printers.addBinding("complex").to(ComplexHelloPrinter.class);	}}@Singletonpublic class Sample {	@Inject	private Map
     
       printers;	public void hello() {		for (String name : printers.keySet()) {			printers.get(name).print();		}	}	public static void main(String[] args) {		Injector injector = Guice.createInjector(new SampleModule());		Sample sample = injector.getInstance(Sample.class);		sample.hello();	}}
     
    

在全世界都沉迷于复杂的Spring框架时，Guice无疑是一股清流，在炎热的夏天，它就像一杯冰爽的橙汁，让人畅快不已。

以上内容还不够看，请移步官方文档https://github.com/google/guice/wiki/GettingStarted