foreword
Java SPI (Service Provider Interface) is an important componentization method, which allows the program to dynamically load some implementation modules at runtime, thereby enhancing the scalability and flexibility of the program. This article will introduce the basic concepts, principles and usage of Java SPI in detail.
1. What is Java SPI
The Java SPI is a standard service discovery mechanism that is widely used in Java. It allows the program to dynamically load the corresponding implementation class through configuration files or annotations at startup, so as to achieve lightweight plug-in development.
2. Principle of Java SPI
The implementation mechanism of Java SPI mainly has three steps:
2.1 Define the interface
First, an interface needs to be defined to describe the implementation method of a certain function. For example:
public interface MyService {
void doSomething();
}
2.2 Implement the interface
Second, the interface needs to be implemented and packaged into a jar package. Each implementation module should contain a configuration file describing the implementation class, the file name is "META-INF/services/interface fully qualified name", for example:
META-INF/services/com.example.MyService
The configuration file should list the full class names of all implementation classes, for example:
com.example.impl1.MyServiceImpl
com.example.impl2.MyServiceImpl
2.3 Load the implementation class
Finally, when the program is running, you can use the default ServiceLoader class to load the implementation class. For example:
ServiceLoader<MyService> loader = ServiceLoader.load(MyService.class);
for (MyService service : loader) {
service.doSomething();
}
In the above code, all classes that implement the MyService interface will be loaded and their doSomething methods will be called in turn.
3. Java SPI example
A simple example is given below to show how to use the Java SPI mechanism to realize lightweight plug-in development. First, define a simple interface Plugin:
public interface Plugin {
void execute();
}
Then, write two implementation classes PluginImplA and PluginImplB:
public class PluginImplA implements Plugin {
public void execute() {
System.out.println("PluginImplA.execute()");
}
}
public class PluginImplB implements Plugin {
public void execute() {
System.out.println("PluginImplB.execute()");
}
}
Next, create a file named "Plugin" in the "META-INF/services" directory with the following content:
com.example.spi.PluginImplA
com.example.spi.PluginImplB
Add the following code to the project to run and view the results:
public class Main {
public static void main(String[] args) {
ServiceLoader<Plugin> loader = ServiceLoader.load(Plugin.class);
for (Plugin plugin : loader) {
plugin.execute();
}
}
}
operation result:
PluginImplA.execute()
PluginImplB.execute()
4. Summary
Java SPI is a lightweight component development method, which allows the program to dynamically load the implementation class at runtime, thereby enhancing the scalability and flexibility of the program. The implementation principle of Java SPI is relatively simple. You only need to define the interface, implement the interface and package it into a jar package, and then list the complete class name of the implementation class in the configuration file. The advantage of Java SPI is that it is easy to use, applicable to various scenarios, and does not require additional dependent libraries.