https://github.com/mfvitale/java-service-loader-examples

An example to show how Java SPI can benefit from Java modules
https://github.com/mfvitale/java-service-loader-examples

Last synced: 26 days ago
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An example to show how Java SPI can benefit from Java modules

• Host: GitHub
• URL: https://github.com/mfvitale/java-service-loader-examples
• Owner: mfvitale
• Created: 2024-01-22T16:21:29.000Z (10 months ago)
• Default Branch: main
• Last Pushed: 2024-01-26T11:43:13.000Z (10 months ago)
• Last Synced: 2024-01-26T13:14:36.241Z (10 months ago)
• Language: Java
• Size: 9.77 KB
• Stars: 0
• Watchers: 1
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
# Benefits from using Java Modules with Java Service Provider Interface

Suppose that you have a two simple interfaces 

```java

package me.mfvitale.spi;

public interface Printer {

    void print();

}

```

```java

package me.mfvitale.spi;

public interface Scanner {

    void scan();

}

```

And you want to use `ServiceLoader` to dinamically load implementations. You can do it in the following way

```java

Printer printer = ServiceLoader.load(Printer.class).findFirst()

                .orElseThrow();

Scanner scanner = ServiceLoader.load(Scanner.class).findFirst()

                .orElseThrow();

```

Now we can provide a class that implements both beahavior 

```java

package me.mfvitale;

import me.mfvitale.spi.Printer;

import me.mfvitale.spi.Scanner;

public class AllInOnePrinter implements Printer, Scanner {

    @Override

    public void print() {

        System.out.println("Printing from " + this);

    }

    @Override

    public void scan() {

        System.out.println("Scanning from " + this);

    }

}

```

All ssems good right? Actually, when you ask to Java Service Loader first `Printer` and then `Scanner`, even if you have a sigle implementation for both interfaces, you will get two different instances. 

Starting from Java 9 there is a solution that uses the Java Module. 

From [java.util.ServiceLoader API docs:](https://docs.oracle.com/javase/9/docs/api/java/util/ServiceLoader.html)

> A service provider that is developed in a module has no control over when it is instantiated, since that occurs at the behest of the application, but it does have control over how it is instantiated:

> If the service provider declares a provider method, then the service loader invokes that method to obtain an instance of the service provider. A provider method is a public static method named "provider" with no formal parameters and a return type that is assignable to the service's interface or class.

> In this case, the service provider itself need not be assignable to the service's interface or class.

With this approach we can implement our `AllInOnePrinter` in this way

```java

package io.mfvitale;

import me.mfvitale.spi.Printer;

import me.mfvitale.spi.Scanner;

public class AllInOnePrinter implements Printer, Scanner {

    static final AllInOnePrinter INSTANCE = new AllInOnePrinter();

    private AllInOnePrinter() {}

    public static AllInOnePrinter provider() {

        return INSTANCE;

    }

    @Override

    public void print() {

        System.out.println("Printing from " + this);

    }

    @Override

    public void scan() {

        System.out.println("Scanning from " + this);

    }

}

```

and the we need to provide our `module-info.java`

```java

module io.mfvitale.extension {

    requires me.mfvitale.api;

    exports io.mfvitale;

    provides Scanner with AllInOnePrinter;

    provides Printer with AllInOnePrinter;

}

```