The SPI mechanism of Java with a simple example

One, SPI mechanism

Here is a concept of SPI, SPI English for service Provider interface is literally understood as the service provider interface, as in the name of the SPI to understand the SPI is the service provider interface My definition of SPI: an interface that is available to developers of service providers and extended framework functions.

In our daily development is to abstract the problem into the API and then provide the implementation of various APIs, the implementation of these APIs are encapsulated with our jar or framework, although when we want to provide a new implementation of the API can not modify the original code can only implement the API to provide a new implementation of the API, But we're still building a new jar or framework (although it's possible to scan a directory for a new implementation of the API's loaded APIs, but this is not a Java mechanism, it's just the Hack method), and with the Java SPI mechanism we can provide a new implementation for the API without modifying the jar package or framework.

Many frameworks use Java's SPI mechanism, such as Java.sql.Driver's SPI implementation (MySQL driver, Oracle driver, etc.), common-logging log interface Implementation, Dubbo extension implementation, and so on.

SPI the Convention of the Mechanism:

1) Create a file in the meta-inf/services/directory named after the interface fully qualified name the full qualified name of the API-specific implementation class for the file content

2) using the Serviceloader class to dynamically load the implementation class in the Meta-inf

3) If the SPI implementation class is a jar, it needs to be placed in the main program Classpath

4) API specific implementation class must have a constructor without parameters

SPI mechanism structure diagram

Second, SPI Mechanism Example

Instance structure diagram

ioperation Interface:

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/**   * Created by LX on 2015/3/8.   */ public  interface   ioperation {       Code class= "Java keyword" >public  int   operation ( int   numbera,  int  numberb);

Plusoperationimpl implementation:

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importco.solinx.demo.api.IOperation;/** * Created by LX on 2015/3/8. */publicclassPlusOperationImpl implementsIOperation {     @Override    publicintoperation(intnumberA, intnumberB) {        returnnumberA + numberB;    }}

SPI implementation class for the interface: Divisionoperationimpl

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importco.solinx.demo.api.IOperation;/** * Created by LX on 2015/3/8. */publicclassDivisionOperationImpl implementsIOperation {     @Override    publicintoperation(intnumberA, intnumberB) {        returnnumberA / numberB;    }}

meta-inf/services files in the directory:

File name: co.solinx.demo.api.IOperation, Content: Co.solinx.demo.spi.DivisionOperationImpl

Main class:

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importco.solinx.demo.api.IOperation;importco.solinx.demo.impl.PlusOperationImpl;importco.solinx.demo.spi.DivisionOperationImpl;import java.util.Iterator;importjava.util.ServiceLoader;/** * Created by LX on 2015/3/8. */publicclassmain {    publicstatic voidmain(String[] args) {        IOperation plus = newPlusOperationImpl();        IOperation division = newDivisionOperationImpl();        System.out.println(plus.operation(5, 3));        System.out.println(division.operation(9, 3));        ServiceLoader<IOperation> operations = ServiceLoader.load(IOperation.class);        Iterator<IOperation> operationIterator = operations.iterator();        System.out.println("classPath:"+System.getProperty("java.class.path"));        while(operationIterator.hasNext()) {            IOperation operation = operationIterator.next();            System.out.println(operation.operation(6, 3));        }    }}

Operation Result:

if the SPI implementing packaging as a jar need to put the jar put it in Classpath . directory, the SPI jar package Run Result:

The SPI mechanism of Java with a simple example