Detailed explanation of using maven to build a java9 service instance

sequence

This article mainly studies how to build java9 multi module and service examples in maven

maven

The entire project is the same as the traditional maven multi-module engineering structure. A module in java9 corresponds to a module in the maven project. Here is the pom file in the root directory:

 <?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>com.example</groupId> <artifactId>java9-service-demo</artifactId> <version>0.0.1-SNAPSHOT</version> <modules> <module>consumer-demo</module> <module>service-sort</module> <module>service-sort</module> <module>service-sort-bubble</module> <module>service-sort-merge</module> </modules> <packaging>pom</packaging> <properties> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> <!--Let Intellij compile java9 correctly, otherwise it will always be changed back to use 1.5--> <maven.compiler.source>9</maven.compiler.source> <maven.compiler.target>9</maven.compiler.target> </properties> <build> <pluginManagement> <plugins> <plugins> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.6.1</version> <configuration> <release>9</release> </configuration> </plugins> </pluginManagement> </build></project>

Here is a maven-compiler-plugin managed, and the release is configured to be 9. Because java9 supports multi release, it can support multiple Java versions at the same time. It is compiled as java9 version here.

service-sort

This is the service interface module

 module service.sort { exports service.sort; uses service.sort.SortService;}

Here, the use SortService is declared that it needs to use ServiceLoader to load the service instance in this module.

 public interface SortService { public <T extends Comparable> List<T> sortList(List<T> list); public static SortService getProviderInstanceLazy() { Stream<Provider<SortService>> providers = ServiceLoader.load(SortService.class) .stream(); //provider method will not instantiate until get SortService service = providers.map(Provider::get) .findAny() .orElse(null); return service; }}

While declaring the interface, static methods are also added to load service instances.

service-sort-bubble

maven

 <?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <parent> <artifactId>java9-service-demo</artifactId> <groupId>com.example</groupId> <version>0.0.1-SNAPSHOT</version> </parent> <modelVersion>4.0.0</modelVersion> <groupId>com.example</groupId> <artifactId>service-sort-bubble</artifactId> <packaging>jar</packaging> <dependencies> <dependency> <groupId>com.example</groupId> <artifactId>service-sort</artifactId> <version>0.0.1-SNAPSHOT</version> </dependency> </dependencies></project>

Add dependencies on the API package here

module-info.java

 module service.sort.bubble { require service.sort; provide service.sort.SortService with sort.impl.bubble.BubbleSort;}

Here it is stated that BubbleSort provides the implementation of SortService

BubbleSort

 public class BubbleSort implements SortService { public <T extends Comparable> List<T> sortList(List<T> list) { System.out.println("use BubbleSort"); for (int outer = 0; outer < list.size() - 1; outer++) { for (int inner = 0; inner < list.size()-outer-1; inner++) { if (list.get(inner).compareTo(list.get(inner + 1)) > 0) { swap(list, inner); } } } return list; } private <T> void swap(List<T>list, int inner) { T temp = list.get(inner); list.set(inner, list.get(inner + 1)); list.set(inner + 1, temp); }}

service-sort-merge

maven

 <?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <parent> <artifactId>java9-service-demo</artifactId> <groupId>com.example</groupId> <version>0.0.1-SNAPSHOT</version> </parent> <modelVersion>4.0.0</modelVersion> <groupId>com.example</groupId> <artifactId>service-sort-merge</artifactId> <packaging>jar</packaging> <dependencies> <dependency> <groupId>com.example</groupId> <artifactId>service-sort</artifactId> <version>0.0.1-SNAPSHOT</version> </dependency> </dependencies></project>

module-info.java

 module service.sort.merge { require service.sort; provide service.sort.SortService with sort.impl.merge.MergeSort;}

Here is the implementation of MergeSort as the SortService interface

MergeSort

 import java.util.List;import java.util.ArrayList;import java.util.Collections;import java.util.Arrays;import service.sort.SortService;public class MergeSort implements SortService { public <T extends Comparable> List<T> sortList(List<T> list) { System.out.println("using MergeSort"); Collections.sort(list); return list; }}

Consumer

maven

 <?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <parent> <artifactId>java9-service-demo</artifactId> <groupId>com.example</groupId> <version>0.0.1-SNAPSHOT</version> </parent> <modelVersion>4.0.0</modelVersion> <groupId>com.example</groupId> <artifactId>consumer-demo</artifactId> <packaging>jar</packaging> <dependencies> <dependency> <groupId>com.example</groupId> <artifactId>service-sort</artifactId> <version>0.0.1-SNAPSHOT</version> </dependency> </dependencies></project>

Note that there are no dependencies for implementation classes added here

module-info.java

 module consumer { require service.sort;}

Main

 public class Main { public static void main(String[] args) { System.out.println("sort service consumer started."); List<Integer> data = new ArrayList<Integer>(); data.add(5); data.add(3); data.add(10); data.add(2); data.add(8); SortService sortService = SortService.getProviderInstanceLazy(); if (sortService != null) { sortService.sortList(data); } System.out.println(data); System.out.println("finish"); }}

Compile and run

Compilation

 mvn clean install

This is executed in the root directory

Using bubble

The code copy is as follows:
java --module-path ./consumer-demo/target/consumer-demo-0.0.1-SNAPSHOT.jar:./service-sort/target/service-sort-0.0.1-SNAPSHOT.jar:./service-sort-bubble/target/service-sort-bubble-0.0.1-SNAPSHOT.jar --module consumer/consumer.Main

Note that the jar of bubble to module-path is added here

Output

sort service consumer started.
use BubbleSort
[2, 3, 5, 8, 10]
finish

Use merge

The code copy is as follows:
java --module-path ./consumer-demo/target/consumer-demo-0.0.1-SNAPSHOT.jar:./service-sort/target/service-sort-0.0.1-SNAPSHOT.jar:./service-sort-merge/target/service-sort-merge-0.0.1-SNAPSHOT.jar --module consumer/consumer.Main

Note that merge jar to module-path is added here

Output

sort service consumer started.
using MergeSort
[2, 3, 5, 8, 10]
finish

Both service implementations are added

The code copy is as follows:
java --module-path ./consumer-demo/target/consumer-demo-0.0.1-SNAPSHOT.jar:./service-sort/target/service-sort-0.0.1-SNAPSHOT.jar:./service-sort-bubble/target/service-sort-bubble-0.0.1-SNAPSHOT.jar:./service-sort-merge/target/service-sort-merge-0.0.1-SNAPSHOT.jar --module consumer/consumer.Main

or

The code copy is as follows:
java --module-path ./consumer-demo/target/consumer-demo-0.0.1-SNAPSHOT.jar:./service-sort/target/service-sort-0.0.1-SNAPSHOT.jar:./service-sort-merge/target/service-sort-merge-0.0.1-SNAPSHOT.jar:./service-sort-bubble/target/service-sort-bubble-0.0.1-SNAPSHOT.jar --module consumer/consumer.Main

Output

sort service consumer started.
use BubbleSort
[2, 3, 5, 8, 10]
finish

It seems that it has nothing to do with the order of adding to the path. Even if the merge jar package is placed in front of it, it is still a bubble.

summary

In java6, there was already a ServiceLoader, but at that time, it was relied on creating a full path name file for the service interface in the META-INF/services directory of the jar package, and wrote the full path name of the implementation class. After the introduction of modularization, Java9 also supports the declaration of service provider and consumer information in module-info.java. In this way, the module system can support ServiceLoader without using the original META-INF declaration method.

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