Dagger 2 example

build.sh

javac -cp  $M2_HOME/repository/com/google/dagger/dagger/2.0/dagger-2.0.jar:$M2_HOME/repository/javax/inject/javax.inject/1/javax.inject-1.jar:$M2_HOME/repository/com/google/dagger/dagger-compiler/2.0/dagger-compiler-2.0.jar:$M2_HOME/repository/com/google/dagger/dagger-producers/2.0-beta/dagger-producers-2.0-beta.jar:$M2_HOME/repository/com/google/guava/guava/18.0/guava-18.0.jar -d . *.java

Dagger 2 example.md

This example contains initially 3 java files:

• SearchApp.java Has the main method
• SearchInPage.java Perfoms the search of a keyword in a string
• HttpClient.java Fetch a webpage

SearchInPage has a dependency on HttpClient, this example is to show how Dagger 2 can be used as Dependency Injection framework.

With Dagger 2 we change the dependency by adding the @Inject annotation, and we add 2 more clases: a Module and a Component which will be used by Dagger 2 to generate the required code to satisfy the dependency injection

HttpClient.java

package app;

import java.net.*;
import java.io.*;

public class HttpClient {

    /**
      * Fetch the given URI and returns a string with the content.
     */ 
    public String doGet(String uri) {
        try {
            URL url = new URL(uri);
            InputStream in = url.openConnection().getInputStream();
            byte[] buffer = new byte[1024];
            ByteArrayOutputStream os = new ByteArrayOutputStream();
            for( int len; ( len = in.read(buffer)) != -1 ;) {
                os.write(buffer, 0 , len );
            }
            os.flush();
            return new String(os.toByteArray());
        } catch ( Exception e ) {
            throw new RuntimeException(e);
        }
    }
}

run.sh

java -classpath .:$M2_HOME/repository/com/google/dagger/dagger/2.0/dagger-2.0.jar:$M2_HOME/repository/javax/inject/javax.inject/1/javax.inject-1.jar:$M2_HOME/repository/com/google/dagger/dagger-compiler/2.0/dagger-compiler-2.0.jar:$M2_HOME/repository/com/google/dagger/dagger-producers/2.0-beta/dagger-producers-2.0-beta.jar:$M2_HOME/repository/com/google/guava/guava/18.0/guava-18.0.jar app.SearchApp

SearchApp.java

package app;

public class SearchApp {
        public static void main( String ... args ) {
        SearchInPage a = DaggerSearchComponent.create().searchInPage(); 
        for ( String s : a.search("domains") ) {
            System.out.println(s);
        }
    }
} 

SearchComponent.java

package app;

import dagger.Component;

@Component( modules = SearchModule.class )
public interface SearchComponent {
    SearchInPage searchInPage();
}
    

SearchInPage.java

package app;

import java.util.*;

import javax.inject.Inject;

public class SearchInPage {
    @Inject HttpClient client;
    @Inject String host;

    @Inject
    public SearchInPage() {}

    /**
     * Search for the given keyword in the page
     */
    public List<String> search( String keywork ) {
        String content = client.doGet(host);
        List<String> result = new ArrayList<String>();
        StringBuilder wordBuffer = new StringBuilder();
        // iterates the content
        // and test if a word matches the keyword
        // when a white space is found
        for( int i = 0 ; i < content.length() ; i++ ) {
            char c = content.charAt(i);
            if (Character.isWhitespace(c) ) {
                String word = wordBuffer.toString();
                if ( word.equals(keywork) ) {
                    result.add(word);
                }
                wordBuffer.setLength(0);
            } else {
                wordBuffer.append(c);
            }
        }
        return result;
    }
}

SearchModule.java

package app; 

import dagger.Module;
import dagger.Provides;

@Module
public class SearchModule {

    @Provides HttpClient provideHttpClient() {
        return new HttpClient();
    }
    @Provides String provideHost() {
        return "http://www.iana.org/domains/reserved";
    }
}