Google Guice - Quick Guide

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Google Guice - Overview

Guice is an open source, Java-based dependency injection framework. It is lightweight and is developed as well as managed by Google. This chapter will give you an overview of Guice framework.

Dependency Injection

Every Java-based application has a few objects that work together to present what the end-user sees as a working application. When writing a complex Java application, application classes should be as independent as possible of other Java classes to increase the possibility to reuse these classes and to test them independently of other classes while unit testing. Dependency Injection (or sometime called wiring) helps in gluing these classes together and at the same time keeping them independent.

Consider you have an application which has a text editor component and you want to provide a spell check. Your standard code would look something like this −

public class TextEditor {
   private SpellChecker spellChecker;
   
   public TextEditor() {
      spellChecker = new SpellChecker();
   }
}

Note that here we have created a dependency between the TextEditor and the SpellChecker. In an inversion of control scenario, we would instead do something like this −

public class TextEditor {
   private SpellChecker spellChecker;
   
   @Inject
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
}

Here, the TextEditor should not worry about SpellChecker implementation. The SpellChecker will be implemented independently and will be provided to the TextEditor at the time of TextEditor instantiation.

Dependency Injection using Guice (Binding)

Dependency Injection is controlled by the Guice Bindings. Guice uses bindings to map object types to their actual implementations. These bindings are defined a module. A module is a collection of bindings as shown below −

public class TextEditorModule extends AbstractModule {
   @Override 
   protected void configure() {
      /*
         * Bind SpellChecker binding to WinWordSpellChecker implementation 
         * whenever spellChecker dependency is used.
      */
      bind(SpellChecker.class).to(WinWordSpellChecker.class);
   }
}

An injector is the object-graph builder and a Module is its core building block. Thus, the first step is to create an injector and then use the injector to get the objects.

public static void main(String[] args) {
   /*
      * Guice.createInjector() takes Modules, and returns a new Injector
      * instance. This method is to be called once during application startup.
   */
   
   Injector injector = Guice.createInjector(new TextEditorModule());
   /*
      * Build object using injector
   */
   TextEditor textEditor = injector.getInstance(TextEditor.class);   
}

In the above example, TextEditor class object graph is constructed by Guice and this graph contains TextEditor object and its dependency as WinWordSpellChecker object.

Google Guice - Environment Setup

Let us look into setting up the environment for Guice, before proceeding into its functionalities.

Local Environment Setup

This section guides you on how to download and set up Java environment on your machine. Please follow the steps mentioned below to set up the environment.

Java SE is freely available from the link Download Java. Download a version based on your operating system. Then, follow the instructions to download Java and run the .exe file to install Java on your machine. Once you installed Java, you would need to set environment variables to point to correct installation directories.

Setting up the Path for Windows 2000/XP

We are assuming that you have installed Java in c:\Program Files\java\jdk directory −

  • Right-click on 'My Computer' and select 'Properties'.

  • Click on the 'Environment variables' button under the 'Advanced' tab.

  • Now, alter the 'Path' variable so that it also contains the path to the Java executable. Example, if the path is currently set to 'C:\WINDOWS\SYSTEM32', then change your path to read 'C:\WINDOWS\SYSTEM32;c:\Program Files\java\jdk\bin'.

Setting up the Path for Windows 95/98/ME

In case, you have installed Java in c:\Program Files\java\jdkdirectory, edit the 'C:\autoexec.bat' file and add the following line at the end: 'SET PATH=%PATH%;C:\Program Files\java\jdk\bin'.

Setting up the Path for Linux, UNIX, Solaris, FreeBSD

Environment variable PATH should be set to point to where the Java binaries have been installed. You may refer to your shell documentation for more information.

For example, if you use bash as your shell, then you would add the following line to the end: '.bashrc: export PATH=/path/to/java:$PATH'

Popular Java Editors

To write your Java programs, you need a text editor. There are many sophisticated IDEs available in the market. But for now, you can consider one of the following −

  • Notepad − On Windows machine you can use any simple text editor like Notepad (Recommended for this tutorial), TextPad.

  • Netbeans − It is a Java IDE that is open-source and free which can be downloaded from https://www.netbeans.org/index.html.

  • Eclipse − It is also a Java IDE developed by the eclipse open-source community and can be downloaded from https://www.eclipse.org/.

Google Guice Environment

Download the latest version of Google Guice and related jar files.

At the time of writing this tutorial, we have copied them into C:\>Google folder.

OS Archive name
Windows guice-4.1.0.jar;aopalliance-1.0.jar;guava-16.0.1.jar;javax.inject-1.jar
Linux guice-4.1.0.jar;aopalliance-1.0.jar;guava-16.0.1.jar;javax.inject-1.jar
Mac guice-4.1.0.jar;aopalliance-1.0.jar;guava-16.0.1.jar;javax.inject-1.jar

Set CLASSPATH Variable

Set the CLASSPATH environment variable to point to the Guice jar location. Assuming, you have stored Guice and related jars in Google folder on various Operating Systems as follows.

OS Output
Windows Set the environment variable CLASSPATH to %CLASSPATH%;C:\Google\guice-4.1.0.jar;C:\Google\aopalliance-1.0.jar;C:\Google\guava-16.0.1.jar;C:\Google\javax.inject-1.jar;.;
Linux export CLASSPATH=$CLASSPATH:Google/guice-4.1.0.jar:Google/aopalliance-1.0.jar:Google/guava-16.0.1.jar:Google/javax.inject-1.jar:.
Mac export CLASSPATH=$CLASSPATH:Google/guice-4.1.0.jar:Google/aopalliance-1.0.jar:Google/guava-16.0.1.jar:Google/javax.inject-1.jar:.

Google Guice - First Application

Let's create a sample console based application where we'll demonstrate dependency injection using Guice binding mechanism step by step.

Step 1 − Create Interface

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

Step 2 − Create Implementation

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   @Override
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   } 
}

Step 3 − Create Bindings Module

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   protected void configure() {
      bind(SpellChecker.class).to(SpellCheckerImpl.class);
   } 
}

Step 4 − Create Class with dependency

class TextEditor {
   private SpellChecker spellChecker;
   
   @Inject
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   }
}

Step 5 − Create Injector

Injector injector = Guice.createInjector(new TextEditorModule());

Step 6 − Get Object with dependency fulfilled

TextEditor editor = injector.getInstance(TextEditor.class);

Step 7 − Use the object

editor.makeSpellCheck();

Complete Example

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck(); 
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   }
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(SpellChecker.class).to(SpellCheckerImpl.class);
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   @Override
   
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   } 
}

Output

Compile and run the file, you will see the following output.

Inside checkSpelling.

Google Guice - Linked Binding

In Linked bindings, Guice maps a type to its implementation. In example discussed below, we have mapped SpellChecker interface with its implementation SpellCheckerImpl.

bind(SpellChecker.class).to(SpellCheckerImpl.class);

We can also mapped the concrete class to its subclass. See the example below −

bind(SpellCheckerImpl.class).to(WinWordSpellCheckerImpl.class);

Observe that here we have chained the bindings. Let us see the result in complete example.

Complete Example

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck(); 
   } 
}
class TextEditor {
   private SpellChecker spellChecker;
   @Inject
   
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   }
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(SpellChecker.class).to(SpellCheckerImpl.class);
      bind(SpellCheckerImpl.class).to(WinWordSpellCheckerImpl.class);
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   @Override
   
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   } 
}

//subclass of SpellCheckerImpl
class WinWordSpellCheckerImpl extends SpellCheckerImpl {
   @Override
   
   public void checkSpelling() {
      System.out.println("Inside WinWordSpellCheckerImpl.checkSpelling." );
   } 
}

Output

Compile and run the file, you will see the following output.

Inside WinWordSpellCheckerImpl.checkSpelling.

Google Guice - Binding Annotations

As we can bind a type with its implementation. In case we want to map a type with multiple implementations, we can create custom annotation as well. See the below example to understand the concept.

Create a binding annotation

@BindingAnnotation @Target({ FIELD, PARAMETER, METHOD }) @Retention(RUNTIME)
@interface WinWord {}

  • @BindingAnnotation − Marks annotation as binding annotation.

  • @Target − Marks applicability of annotation.

  • @Retention − Marks availability of annotation as runtime.

Mapping using binding annotation

bind(SpellChecker.class).annotatedWith(WinWord.class).to(WinWordSpellCheckerImpl.class);

Inject using binding annotation

@Inject
public TextEditor(@WinWord SpellChecker spellChecker) {
   this.spellChecker = spellChecker;
}

Complete Example

Create a java class named GuiceTester.

GuiceTester.java

import java.lang.annotation.Target;

import com.google.inject.AbstractModule;
import com.google.inject.BindingAnnotation;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;

import java.lang.annotation.Retention;

import static java.lang.annotation.RetentionPolicy.RUNTIME;
import static java.lang.annotation.ElementType.PARAMETER;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;

@BindingAnnotation @Target({ FIELD, PARAMETER, METHOD }) @Retention(RUNTIME)
@interface WinWord {}

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck();     
   } 
}
class TextEditor {
   private SpellChecker spellChecker;   
   @Inject
   
   public TextEditor(@WinWord SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling(); 
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(SpellChecker.class).annotatedWith(WinWord.class)
         .to(WinWordSpellCheckerImpl.class);    
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   @Override
   
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   } 
}

//subclass of SpellCheckerImpl
class WinWordSpellCheckerImpl extends SpellCheckerImpl {
   @Override
   
   public void checkSpelling() {
      System.out.println("Inside WinWordSpellCheckerImpl.checkSpelling." );
   } 
}

Output

Now, compile and run the file. You can see the following output −

Inside WinWordSpellCheckerImpl.checkSpelling.

Google Guice - @Named Binding

Guice provides another way also to map bindings without creating a custom annotation. It allows so using @Named annotation.

Mapping using named annotation

bind(SpellChecker.class).annotatedWith(Names.named("OpenOffice")).to(OpenOfficeWordSpellCheckerImpl.class);

Inject using @Named annotation

@Inject
public TextEditor(@Named("OpenOffice") SpellChecker spellChecker) {
   this.spellChecker = spellChecker;
}

Complete Example

Create a java class named GuiceTester.

GuiceTester.java

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.Named;
import com.google.inject.name.Names;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck();
   } 
}
class TextEditor {
   private SpellChecker spellChecker;
   
   @Inject
   public TextEditor(@Named("OpenOffice") SpellChecker spellChecker) {
      this.spellChecker = spellChecker;      
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling(); 
   }  
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(SpellChecker.class).annotatedWith(Names.named("OpenOffice"))
         .to(OpenOfficeWordSpellCheckerImpl.class);
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   @Override
   
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   } 
}

//subclass of SpellCheckerImpl
class OpenOfficeWordSpellCheckerImpl extends SpellCheckerImpl {
   @Override
   
   public void checkSpelling() {
      System.out.println("Inside OpenOfficeWordSpellCheckerImpl.checkSpelling." );
   } 
}

Output

Now, compile and run the file. You can see the following output −

Inside OpenOfficeWordSpellCheckerImpl.checkSpelling.

Google Guice - Constant Bindings

Guice provides a way to create bindings with value objects or constants. Consider the case where we want to configure JDBC url.

Inject using @Named annotation

@Inject
public void connectDatabase(@Named("JBDC") String dbUrl) {
   //...
}

This can be achived using toInstance() method.

bind(String.class).annotatedWith(Names.named("JBDC")).toInstance("jdbc:mysql://localhost:5326/emp");

Complete Example

Create a java class named GuiceTester.

GuiceTester.java

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.Named;
import com.google.inject.name.Names;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeConnection();
   } 
}
class TextEditor {
   private String dbUrl;
   
   @Inject
   public TextEditor(@Named("JDBC") String dbUrl) {
      this.dbUrl = dbUrl;
   }
   public void makeConnection() {
      System.out.println(dbUrl);
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(String.class)
         .annotatedWith(Names.named("JDBC"))
         .toInstance("jdbc:mysql://localhost:5326/emp");
   } 
}

Output

Now, compile and run the file. You can see the following output −

jdbc:mysql://localhost:5326/emp

Google Guice - @Provides Annotation

Guice provides a way to create bindings with complex objects using @provides method.

@Provides
public SpellChecker provideSpellChecker() {
   String dbUrl = "jdbc:mysql://localhost:5326/emp";
   String user = "user";
   int timeout = 100;
   SpellChecker SpellChecker = new SpellCheckerImpl(dbUrl, user, timeout);
   return SpellChecker;
}

This methods is being part of Binding Module and provides the complex object to be mapped. See the complete example below.

Complete Example

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.Provides;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck();
   } 
}
class TextEditor {
   private SpellChecker spellChecker;
   
   @Inject
   public TextEditor( SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   protected void configure() {} 

   @Provides
   public SpellChecker provideSpellChecker() {
      String dbUrl = "jdbc:mysql://localhost:5326/emp";
      String user = "user";
      int timeout = 100;

      SpellChecker SpellChecker = new SpellCheckerImpl(dbUrl, user, timeout);
      return SpellChecker;
   }
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {

   private String dbUrl;
   private String user;
   private Integer timeout;

   @Inject
   public SpellCheckerImpl(String dbUrl, 
      String user, 
      Integer timeout) {
      this.dbUrl = dbUrl;
      this.user = user;
      this.timeout = timeout;
   } 
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
      System.out.println(dbUrl);
      System.out.println(user);
      System.out.println(timeout);
   }
}

Output

Now, compile and run the file. You can see the following output −

Inside checkSpelling.
jdbc:mysql://localhost:5326/emp
user
100

Google Guice - Provider Class

As @provides method becomes more complex, this method can be moved to separate classes using Provider interface.

class SpellCheckerProvider implements Provider<SpellChecker> {
   @Override
   public SpellChecker get() {
      String dbUrl = "jdbc:mysql://localhost:5326/emp";
      String user = "user";
      int timeout = 100;
      SpellChecker SpellChecker = new SpellCheckerImpl(dbUrl, user, timeout);
      return SpellChecker;
   } 
}

Next, you have to map the provider to type.

bind(SpellChecker.class).toProvider(SpellCheckerProvider.class);

See the complete example below.

Complete Example

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.Provider;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck();
   } 
}
class TextEditor {
   private SpellChecker spellChecker;
   
   @Inject
   public TextEditor( SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(SpellChecker.class).toProvider(SpellCheckerProvider.class);
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {

   private String dbUrl;
   private String user;
   private Integer timeout;

   @Inject
   public SpellCheckerImpl(String dbUrl, 
      String user, 
      Integer timeout) {
      this.dbUrl = dbUrl;
      this.user = user;
      this.timeout = timeout;
   } 
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
      System.out.println(dbUrl);
      System.out.println(user);
      System.out.println(timeout);
   }
}
class SpellCheckerProvider implements Provider<SpellChecker> {
   @Override
   
   public SpellChecker get() {
      String dbUrl = "jdbc:mysql://localhost:5326/emp";
      String user = "user";
      int timeout = 100;

      SpellChecker SpellChecker = new SpellCheckerImpl(dbUrl, user, timeout);
      return SpellChecker;
   }
}

Output

Now, compile and run the file. You can see the following output −

Inside checkSpelling.
jdbc:mysql://localhost:5326/emp
user
100

Google Guice - Constructor Bindings

Guice provides a way to create bindings with specific constructor of an object using toConstructor() method.

@Override
protected void configure() {
   try {
      bind(SpellChecker.class)
         .toConstructor(SpellCheckerImpl.class.getConstructor(String.class));
   } catch (NoSuchMethodException | SecurityException e) {
      System.out.println("Required constructor missing");
   } 
}

See the complete example below.

Complete Example

Create a java class named GuiceTester.

GuiceTester.java

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.Named;
import com.google.inject.name.Names;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck();
   } 
}

class TextEditor {
   private SpellChecker spellChecker;
   
   @Inject
   public TextEditor( SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      try {
         bind(SpellChecker.class)
            .toConstructor(SpellCheckerImpl.class.getConstructor(String.class));
      } catch (NoSuchMethodException | SecurityException e) {
         System.out.println("Required constructor missing");
      } 
      bind(String.class)
         .annotatedWith(Names.named("JDBC"))
         .toInstance("jdbc:mysql://localhost:5326/emp");
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   private String dbUrl;
   public SpellCheckerImpl(){}

   public SpellCheckerImpl(@Named("JDBC") String dbUrl) {
      this.dbUrl = dbUrl;
   } 
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
      System.out.println(dbUrl); 
   }
}

Output

Now, compile and run the file. You can see the following output −

Inside checkSpelling.
jdbc:mysql://localhost:5326/emp

Google Guice - Inbuilt Bindings

Guice provides inbuilt binding for java.util.logging.Logger class. Logger's name is automatically set to the name of the class into which the Logger is injected. See the example below.

Example

Create a java class named GuiceTester.

GuiceTester.java

import java.util.logging.Logger;

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck();
   } 
}
class TextEditor {
   private Logger logger;

   @Inject
   public TextEditor( Logger logger) {
      this.logger = logger;
   }
   public void makeSpellCheck() {
      logger.info("In TextEditor.makeSpellCheck() method");
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {

   @Override
   protected void configure() {
   } 
}

Output

Now, compile and run the file. You can see the following output −

Dec 20, 2017 12:51:05 PM TextEditor makeSpellCheck
INFO: In TextEditor.makeSpellCheck() method

Google Guice - Just-in-time Bindings

As bindings are defined in Binding Module, Guice uses them whenever it needs to inject dependencies. In case bindings are not present, it can attempt to create just-in-time bindings. Bindings present in the binding module are called Explicit bindings and are of higher precedence whereas just-in-time bindings are called Implicit bindings. Note that if both type of bindings are present, explicit bindings are considered for mapping.

The examples for three types of Just-in-time bindings are given below −

Sr.No. Binding Type & Description
1 Injectable Constructors

Non-private, No-argument constructors are eligible for just-in-time bindings. Another way is to annotate a constructor with @Inject annotation.
2 @ImplementatedBy annotation

@ImplementatedBy annotation tells the guice about the implementation class. No binding is required in Binding Module in such a case.
3 @ProvidedBy annotation

@ProvidedBy annotation tells the guice about the provider of implementation class. No binding is required in Binding Module in such a case.

Google Guice - Constructor Injection

Injection is a process of injecting dependency into an object. Constructor injection is quite common. In this process, dependency is injected as argument to the constructor. See the example below.

Example

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck(); 
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   }
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(SpellChecker.class).to(SpellCheckerImpl.class);
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   @Override
   
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   } 
}

Output

Now, compile and run the file. You can see the following output −

Inside checkSpelling.

Google Guice - Method Injection

Injection is a process of injecting dependency into an object. Method injection is used to set value object as dependency to the object. Observe the example given below.

Example

Create a java class named GuiceTester.

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.ImplementedBy;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.name.Named;
import com.google.inject.name.Names;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck();
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public TextEditor( SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() { 
      bind(String.class)
         .annotatedWith(Names.named("JDBC"))
         .toInstance("jdbc:mysql://localhost:5326/emp");
   } 
}
@ImplementedBy(SpellCheckerImpl.class)
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   private String dbUrl;
   public SpellCheckerImpl(){}
   
   @Inject 
   public void setDbUrl(@Named("JDBC") String dbUrl) {
      this.dbUrl = dbUrl;
   }
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
      System.out.println(dbUrl); 
   }
}

Output

Now, compile and run the file. You can see the following output −

Inside checkSpelling.
jdbc:mysql://localhost:5326/emp

Google Guice - Field Injection

Injection is a process of injecting dependency into an object. Field injection is used to set value object as dependency to the field of an object. Observe the example given below.

Example

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.ImplementedBy;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.name.Named;
import com.google.inject.name.Names;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck();
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public TextEditor( SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() { 
      bind(String.class)
         .annotatedWith(Names.named("JDBC"))
         .toInstance("jdbc:mysql://localhost:5326/emp");
   } 
}
@ImplementedBy(SpellCheckerImpl.class)
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   @Inject @Named("JDBC")
   private String dbUrl;
   public SpellCheckerImpl(){}

   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
      System.out.println(dbUrl); 
   }
}

Output

Now, compile and run the file. You can see the following output −

Inside checkSpelling.
jdbc:mysql://localhost:5326/emp

Google Guice - Optional Injection

Injection is a process of injecting dependency into an object. Optional injection means injecting the dependency if exists. Method and Field injections may be optionally dependent and should have some default value if dependency is not present. See the example below.

Example

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.ImplementedBy;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.name.Named;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck();
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public TextEditor( SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   protected void configure() {} 
}
@ImplementedBy(SpellCheckerImpl.class)
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   private String dbUrl = "jdbc:mysql://localhost:5326/emp";
   public SpellCheckerImpl(){}
   
   @Inject(optional=true)
   public void setDbUrl(@Named("JDBC") String dbUrl) {
      this.dbUrl = dbUrl;
   }
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
      System.out.println(dbUrl); 
   }
}

Output

Now, compile and run the file. You can see the following output −

Inside checkSpelling.
jdbc:mysql://localhost:5326/emp

Google Guice - On-Demand Injection

Injection is a process of injecting dependency into an object. Method and field injections can be used to initialize using exiting object using injector.injectMembers() method. See the example below.

Example

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.ImplementedBy;
import com.google.inject.Inject;
import com.google.inject.Injector;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      SpellChecker spellChecker = new SpellCheckerImpl();
      injector.injectMembers(spellChecker);
      
      TextEditor editor = injector.getInstance(TextEditor.class);     
      editor.makeSpellCheck();
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public void setSpellChecker(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public TextEditor() { }

   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   protected void configure() {      
   } 
}
@ImplementedBy(SpellCheckerImpl.class)
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   public SpellCheckerImpl(){}
   
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
   }
}

Output

Now, compile and run the file. You can see the following output −

Inside checkSpelling.

Google Guice - Scopes

Guice returns a new instance every time it supplies a value as its default behavior. It is configurable via scopes. The various scopes that Guice supports are −

  • @Singleton − Single instance for lifetime of the application. @Singleton object needs to be threadsafe.

  • @SessionScoped − Single instance for a particular session of the web application. @SessionScoped object needs to be threadsafe.

  • @RequestScoped − Single instance for a particular request of the web application. @RequestScoped object does not need to be threadsafe.

Application of Scopes

You can apply scopes in the following ways −

At Class level

@Singleton
class SpellCheckerImpl implements SpellChecker {
   public SpellCheckerImpl(){}
   
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
   }
}

At Configuration level

bind(SpellChecker.class).to(SpellCheckerImpl.class).in(Singleton.class);

At Method level

@Provides @Singleton
public SpellChecker provideSpellChecker() {
   String dbUrl = "jdbc:mysql://localhost:5326/emp";
   String user = "user";
   int timeout = 100;

   SpellChecker SpellChecker = new SpellCheckerImpl(dbUrl, user, timeout);
   return SpellChecker;
}

Example

Let's see the Scope at class level in action.

Result With @Singleton Annotation

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.Singleton;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      SpellChecker spellChecker = new SpellCheckerImpl();
      injector.injectMembers(spellChecker);

      TextEditor editor = injector.getInstance(TextEditor.class);     
      System.out.println(editor.getSpellCheckerId());

      TextEditor editor1 = injector.getInstance(TextEditor.class);     
      System.out.println(editor1.getSpellCheckerId());
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public void setSpellChecker(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public TextEditor() { }

   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   } 
   public double getSpellCheckerId() {
      return spellChecker.getId();
   }
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {   
      bind(SpellChecker.class).to(SpellCheckerImpl.class);
   } 
}
interface SpellChecker {
   public double getId();
   public void checkSpelling();
}
@Singleton
class SpellCheckerImpl implements SpellChecker {
   double id; 
   
   public SpellCheckerImpl() {
      id = Math.random();    
   }
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
   }
   @Override
   public double getId() { 
      return id;
   }
}

Now, compile and run the file, you can see the following output with same numbers.

0.3055839187063575
0.3055839187063575

Result Without @Singleton Annotation

Create a java class named GuiceTester.

GuiceTester.java

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      SpellChecker spellChecker = new SpellCheckerImpl();
      injector.injectMembers(spellChecker);

      TextEditor editor = injector.getInstance(TextEditor.class);     
      System.out.println(editor.getSpellCheckerId());

      TextEditor editor1 = injector.getInstance(TextEditor.class);     
      System.out.println(editor1.getSpellCheckerId());
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public void setSpellChecker(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public TextEditor() { }
   
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   } 
   public double getSpellCheckerId() {
      return spellChecker.getId();
   }
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {   
      bind(SpellChecker.class).to(SpellCheckerImpl.class);
   } 
}
interface SpellChecker {
   public double getId();
   public void checkSpelling();
}
class SpellCheckerImpl implements SpellChecker {
   double id; 
   
   public SpellCheckerImpl() {
      id = Math.random();    
   }
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
   }
   @Override
   public double getId() { 
      return id;
   }
}

Output

Now, compile and run the file. You can see the following output with different numbers −

0.556007079571739
0.22095011760351602

Google Guice - AOP

AOP, Aspect oriented programming entails breaking down program logic into distinct parts called so-called concerns. The functions that span multiple points of an application are called cross-cutting concerns and these cross-cutting concerns are conceptually separate from the application's business logic. There are various common good examples of aspects like logging, auditing, declarative transactions, security, caching, etc.

The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect. Dependency Injection helps you decouple your application objects from each other and AOP helps you decouple cross-cutting concerns from the objects that they affect. AOP is like triggers in programming languages such as Perl, .NET, Java, and others. Guice provides interceptors to intercept an application. For example, when a method is executed, you can add extra functionality before or after the method execution.

Important Classes

  • Matcher − Matcher is an interface to either accept or reject a value. In Guice AOP, we need two matchers: one to define which classes participate, and another for the methods of those classes.

  • MethodInterceptor − MethodInterceptors are executed when a matching method is called. They can inspect the call: the method, its arguments, and the receiving instance. We can perform cross-cutting logic and then delegate to the underlying method. Finally, we may inspect the return value or exception and return.

Example

Create a java class named GuiceTester.

GuiceTester.java

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.matcher.Matchers;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck(); 
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   }
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(SpellChecker.class).to(SpellCheckerImpl.class);
      bindInterceptor(Matchers.any(), 
         Matchers.annotatedWith(CallTracker.class), 
         new CallTrackerService());
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   @Override @CallTracker
   
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   } 
}
@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
@interface CallTracker {}

class CallTrackerService implements MethodInterceptor  {
   @Override
   
   public Object invoke(MethodInvocation invocation) throws Throwable {
      System.out.println("Before " + invocation.getMethod().getName());
      Object result = invocation.proceed();
      System.out.println("After " + invocation.getMethod().getName());
      return result;
   }
}

Output

Compile and run the file, you may see the following output.

Before checkSpelling
Inside checkSpelling.
After checkSpelling
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