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GettingStarted

• User’s Guide
  • Overview
  • Motivation
  • Getting Started
  • Mental Model
  • Scopes
  • Bindings
    • Linked Bindings
    • Binding Annotations
    • Instance Bindings
    • @Provides Methods
    • Provider Bindings
    • Untargeted Bindings
    • Constructor Bindings
    • Built-in Bindings
    • Just-In-Time Bindings
    • Multibindings
    • Restricting the Binding Source
    • Injecting Providers
    • Web and Servlets
      • Getting Started
      • Configuring Guice Servlet
      • Advanced Topics
      • Inspecting Servlet Bindings
      • Getting Started
      • Using JPA
      • Transactions and Units of Work
      • Multiple persistence modules
      • Extending Guice
      • Elements SPI
      • Extensions SPI
      • AssistedInject
      • Custom Scopes
      • Custom Injections
      • Throwing Providers
      • Graphing Guice Applications
      • Testing Support
        • Bound Fields
        • Bootstrap
        • Binding Resolution
        • Injection Points
        • Class Loading
        • Optional AOP
        • Spring Comparison
        • Guice 7.0.0
        • Guice 6.0.0
        • Guice 5.1.0
        • Guice 5.0.1
        • Guice 5.0.0
        • Guice 4.2.3
        • Guice 4.2.2
        • Guice 4.2.1
        • Guice 4.2
        • Guice 4.1
        • Guice 4.0
        • Guice 3.0
        • Guice 2.0
        • Guice 1.0
        • External Documentation
        • Apps that use Guice
        • Discussions

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        How to start doing dependency injection with Guice.

        Guice is a framework that makes it easier for your application to use the dependency injection (DI) pattern. This getting started guide will walk you through a simple example of how you can use Guice to incorporate dependency injection into your application.

        What is dependency injection?

        Dependency injection is a design pattern wherein classes declare their dependencies as arguments instead of creating those dependencies directly. For example, a client that wishes to call a service should not have to know how to construct the service, rather, some external code is responsible for providing the service to the client.

        Here’s a simple example of code that does not use dependency injection:

        class Foo < private Database database; // We need a Database to do some work Foo() < // Ugh. How could I test this? What if I ever want to use a different // database in another application? this.database = new Database("/path/to/my/data"); > >

        The Foo class above creates a fixed Database object directly. This prevents this class from being used with other Database objects and does not allow the real database to be swapped out for a testing database in tests. Instead of writing untestable or inflexible code, you can use dependency injection pattern to address all these issues.

        Here’s the same example, this time using dependency injection:

        class Foo < private Database database; // We need a Database to do some work // The database comes from somewhere else. Where? That's not my job, that's // the job of whoever constructs me: they can choose which database to use. Foo(Database database) < this.database = database; > >

        The Foo class above can be used with any Database objects since Foo has no knowledge of how the Database is created. For example, you can create a test version of Database implementation that uses an in-memory database in tests to make the test hermetic and fast.

        The Motivation page explains why applications should use the dependency injection pattern in more detail.

        Java class constructors that are annotated with @Inject can be called by Guice through a process called constructor injection, during which the constructors’ arguments will be created and provided by Guice.

        Here is an example of a class that uses constructor injection:

        class Greeter < private final String message; private final int count; // Greeter declares that it needs a string message and an integer // representing the number of time the message to be printed. // The @Inject annotation marks this constructor as eligible to be used by // Guice. @Inject Greeter(@Message String message, @Count int count) < this.message = message; this.count = count; > void sayHello() < for (int i=0; i < count; i++) < System.out.println(message); > > >

        In the example above, the Greeter class has a constructor that is called when application asks Guice to create an instance of Greeter . Guice will create the two arguments required, then invoke the constructor. The Greeter class’s constructor arguments are its dependencies and applications use Module to tell Guice how to satisfy those dependencies.

        Applications contain objects that declare dependencies on other objects, and those dependencies form graphs. For example, the above Greeter class has two dependencies (declared in its constructor):

        • A String object for the message to be printed
        • An Integer object for the number of times to print the message

        Guice modules allow applications to specify how to satisfy those dependencies. For example, the following DemoModule configures all the necessary dependencies for Greeter class:

        /** * Guice module that provides bindings for message and count used in * . */ import com.google.inject.Provides; class DemoModule extends AbstractModule < @Provides @Count static Integer provideCount() < return 3; > @Provides @Message static String provideMessage() < return "hello world"; > >

        DemoModule uses the @Provides methods to specify the dependencies.

        In a real application, the dependency graph for objects will be much more complicated and Guice makes creating complex object easy by creating all the transitive dependencies automatically.

        To bootstrap your application, you’ll need to create a Guice Injector with one or more modules in it. For example, a web server application might have a main method that looks like this:

        public final class MyWebServer < public void start() < . >public static void main(String[] args) < // Creates an injector that has all the necessary dependencies needed to // build a functional server. Injector injector = Guice.createInjector( new RequestLoggingModule(), new RequestHandlerModule(), new AuthenticationModule(), new DatabaseModule(), . ); // Bootstrap the application by creating an instance of the server then // start the server to handle incoming requests. injector.getInstance(MyWebServer.class) .start(); > >

        The injector internally holds the dependency graphs described in your application. When you request an instance of a given type, the injector figures out what objects to construct, resolves their dependencies, and wires everything together. To specify how dependencies are resolved, configure your injector with bindings.

        A simple Guice application

        The following is a simple Guice application with all the necessary pieces put together:

        package guicedemo; import static java.lang.annotation.RetentionPolicy.RUNTIME; import com.google.inject.AbstractModule; import com.google.inject.Guice; import com.google.inject.Injector; import com.google.inject.Key; import com.google.inject.Provides; import java.lang.annotation.Retention; import javax.inject.Inject; import javax.inject.Qualifier; public class GuiceDemo < @Qualifier @Retention(RUNTIME) @interface Message <> @Qualifier @Retention(RUNTIME) @interface Count <> /** * Guice module that provides bindings for message and count used in * . */ static class DemoModule extends AbstractModule < @Provides @Count static Integer provideCount() < return 3; > @Provides @Message static String provideMessage() < return "hello world"; > > static class Greeter < private final String message; private final int count; // Greeter declares that it needs a string message and an integer // representing the number of time the message to be printed. // The @Inject annotation marks this constructor as eligible to be used by // Guice. @Inject Greeter(@Message String message, @Count int count) < this.message = message; this.count = count; > void sayHello() < for (int i=0; i < count; i++) < System.out.println(message); > > > public static void main(String[] args) < /* * Guice.createInjector() takes one or more modules, and returns a new Injector * instance. Most applications will call this method exactly once, in their * main() method. */ Injector injector = Guice.createInjector(new DemoModule()); /* * Now that we've got the injector, we can build objects. */ Greeter greeter = injector.getInstance(Greeter.class); // Prints "hello world" 3 times to the console. greeter.sayHello(); > >

        The GuiceDemo application constructed a small dependency graph using Guice that is capable of building instances of Greeter class. Large applications usually have many Module s that can build complex objects.

        Read more on how to conceptualize Guice with a simple mental model.

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        Guice (pronounced ‘juice’) is a lightweight dependency injection framework for Java 8 and above, brought to you by Google.

        License

        google/guice

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        README.md

        • Latest releases:
          • 6.0.0 (supports javax. , mostly supports jakarta.inject )
          • 7.0.0 (supports jakarta. )
          • (6.0.0 & 7.0.0 are equivalent except for their javax/jakarta support.)
          • User Guide,
          • 6.0.0 javadocs
          • 7.0.0 javadocs
          • Latest Snapshot javadocs

          Put simply, Guice alleviates the need for factories and the use of new in your Java code. Think of Guice’s @Inject as the new new. You will still need to write factories in some cases, but your code will not depend directly on them. Your code will be easier to change, unit test and reuse in other contexts.

          Guice embraces Java’s type safe nature. You might think of Guice as filling in missing features for core Java. Ideally, the language itself would provide most of the same features, but until such a language comes along, we have Guice.

          Guice helps you design better APIs, and the Guice API itself sets a good example. Guice is not a kitchen sink. We justify each feature with at least three use cases. When in doubt, we leave it out. We build general functionality which enables you to extend Guice rather than adding every feature to the core framework.

          Guice aims to make development and debugging easier and faster, not harder and slower. In that vein, Guice steers clear of surprises and magic. You should be able to understand code with or without tools, though tools can make things even easier. When errors do occur, Guice goes the extra mile to generate helpful messages.

          For an introduction to Guice and a comparison to new and the factory pattern, see Bob Lee’s video presentation. After that, check out our user’s guide.

          We’ve been running Guice in mission critical applications since 2006, and now you can, too. We hope you enjoy it as much as we do.

          dependency> groupId>com.google.injectgroupId> artifactId>guiceartifactId>   can be 6.0.0, 7.0.0, etc. --> version> version> dependency>

          dependency> groupId>com.google.inject.extensionsgroupId>   can be one of: assistedinject, dagger-adapter, grapher, jmx, jndi, persist, spring, testlib or throwingproviders --> artifactId>guice- artifactId>   must match the guice core version. --> version> version> dependency>

          See Maven Central for more details, including snippets for other build systems such as Gradle, Ivy, sbt, and more.

          

          About

          Guice (pronounced ‘juice’) is a lightweight dependency injection framework for Java 8 and above, brought to you by Google.

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