[case8]使用maven构建java9 service实例

序

本文主要研究下如何在maven里头构建java9 multi module及service实例

maven

整个工程跟传统maven多module的工程结构一样，java9的一个module对应maven project的一个module。下面是根目录下的pom文件：

<?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-bubble</module>
        <module>service-sort-merge</module>
    </modules>
    <packaging>pom</packaging>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <!--让intellij能够正确编译java9，不然老是变回使用1.5-->
        <maven.compiler.source>9</maven.compiler.source>
        <maven.compiler.target>9</maven.compiler.target>
    </properties>

    <build>
        <pluginManagement>
            <plugins>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-compiler-plugin</artifactId>
                    <version>3.6.1</version>
                    <configuration>
                        <release>9</release>
                    </configuration>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>

</project>

这里管理了一个maven-compiler-plugin，配置release为9，因为java9支持multi release，可以同时支持多个java版本，这里编译为java9版本。

service-sort

这个是service接口module

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

这里同时声明uses SortService表示是它需要在这个module里头使用ServiceLoader去加载service实例

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方法等到get的时候才会实例化
        SortService service = providers.map(Provider::get)
                .findAny()
                .orElse(null);
        return service;
    }
}

这里在声明接口的同时，也增加了静态方法，用于加载service实例。

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>

这里添加对api包的依赖

• module-info.java

module service.sort.bubble {
    requires service.sort;
    provides service.sort.SortService with sort.impl.bubble.BubbleSort;
}

这里声明了BubbleSort提供了SortService的实现

• 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 {
    requires service.sort;
    provides service.sort.SortService with sort.impl.merge.MergeSort;
}

这里声明了MergeSort为SortService接口的实现

• 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>

注意这里没有添加实现类的依赖

• module-info.java

module consumer {
    requires 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");
    }
}

编译及运行

• 编译

mvn clean install

这里是在根目录下执行

• 使用bubble

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

注意这里添加了bubble的jar到module-path

输出

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

• 使用merge

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

注意这里添加了merge的jar到module-path

输出

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

• 两个service实现都添加

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

或者

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

输出

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

发现貌似跟添加到path的顺序没有关系，即使把merge的jar包放在前面，也是使用bubble

小结

在java6的时候就已经有ServiceLoader了，不过那个时候是依赖在jar包的META-INF/services目录下创建一个service接口全路径名称的文件，里头写上实现类的全路径名称。java9对在引入模块化后也支持在module-info.java里头声明service的提供方和消费者信息，这样模块系统可以支持ServiceLoader，不需要使用原来的META-INF那种声明方式。

doc

• Slim modular Java 9 runtime Docker image with Alpine Linux
• 如何在Maven项目中设置Java 9