SPI mechanism of Java

Java dictionary 2021-01-22 15:48:53
spi mechanism java


JAVA Of SPI What is the mechanism ?

SPI(Service Provider Interface): ,“ Service provider interface ”, It means in Service user From the point of view “ Interface requirements ”, It's right “ service provider ” The proposed agreement , In short, it is :“ I need this service , Now you're going to satisfy ”.

Does it look very deep . Actually, it's very simple Let's start with an example , It's easier to understand what JAVA Of SPI Mechanism :

First of all, we provide an interface for order processing


/**
* @Author: lty
* @Date: 2021/1/22 14:24
* Order processing interface
*/
public interface OrderHandlerService {
String handler(String orderid);
}

Two implementation classes


public class PddOrderHandler implements OrderHandlerService
{
@Override
public String handler(String orderid) {
System.out.println("Pdd handler execute");
return "Pdd handler execute";
}
}
public class TaobaoOrderHandler implements OrderHandlerService
{
@Override
public String handler(String orderid) {
System.out.println("taobao handler execute");
return "taobao handler execute";
}
}

Class diagram relation :


1. By calling the implementation class directly

Usually in the development process Let's instantiate an object through new The way

public static void main(String[] args) {
OrderHandlerService pddOrderHandler = new PddOrderHandler();
pddOrderHandler.handler("000000001");
}
//Pdd handler execute

2. adopt SPI Provides a way to implement classes

adopt ServiceLoader.load() Method to get the implementation class

public static void main(String[] args) {
// Use spi
ServiceLoader<OrderHandlerService> services = ServiceLoader.load(OrderHandlerService.class);
services.forEach(orderHandlerService -> {
orderHandlerService.handler("000001");
});
//Pdd handler execute
//taobao handler execute
}

Be careful :

adopt SPI The way We need to provide a special document :

The file is located in
/resources/META-INF/services

The file named com.xxx. namely The fully qualified name of the interface .

The content includes two parts The fully qualified name of the implementation class :

com.liangtengyu.service.Impl.PddOrderHandler
com.liangtengyu.service.Impl.TaobaoOrderHandler

The specific underlying implementation can be separated Implement external loading , Each group can also be implemented and SPI The configuration files are packaged into different jar, In the specific use, according to the need to use different jar that will do .

In the source code

ServiceLoader Class defines a prefix
private static final String PREFIX = "META-INF/services/"

It is used to agree on the position specified above , The convention based configuration read looks up here , If we introduce a third party jar package , If jar Medium META-INF/service Yes OrderHandlerService The implementation of the , It will also be read , And instantiate the classes inside .

SPI Application

Here we have JDBC As an example

mysql-connector-java:5.1.32 Bag META-INF/services/ There is a java.sql.Driver

file , The content is :

com.mysql.jdbc.Driver
com.mysql.fabric.jdbc.FabricMySQLDriver

stay SqlLite in There are the same documents .

They are all used to load the implementation java.sql.Driver The location of the interface implementation class

// The content is
org.sqlite.JDBC

Other applications :

Log facade interface implements class loading ,SLF4J Load the log implementation classes of different providers

Spring Used a lot of SPI, such as : Yes servlet3.0 Specification for ServletContainerInitializer The implementation of the 、 Automatic type conversion Type Conversion SPI(Converter SPI、Formatter SPI) etc.

Dubbo It is also widely used in SPI To realize the extension of the framework , But it's right Java Provided native SPI Do the packaging , Allow users to extend the implementation Filter Interface

...

summary

advantage :
Use Java SPI The advantage of mechanism is to realize decoupling , The assembly control logic of the third-party service module is separated from the business code of the caller , Not coupled together . The application can enable framework extension or replace framework components according to the actual business situation .

shortcoming : although ServiceLoader It's also the delay load used , But basically only through traversal all get , That is to say, all the implementation classes of the interface are loaded and instantiated once . If you don't want to use some implementation classes , It's also loaded and instantiated , This creates waste . Get an implementation class , It's not flexible enough , Multiple concurrent multithreading use ServiceLoader An instance of a class is not safe .

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