Java: ServiceLoader Comments

1. Mito

2. Overview

ServiceLoader and Java ClassLoader is the difference between the two and that is mutually interrelated loader .JVM use ClassLoader class will be loaded into memory, this is the first step in a class declaration period (a java class of complete lifecycle experience loaded, connection, initialization, use, and unloaded five stages, of course, there are circumstances after the connection has not been initialized or loaded directly be used).

That ServiceLoader what is it?

ServiceLoader: a simple service provider loading facilities. It is a well-known service interfaces and classes (typically abstract class) set. Service provider is a specific implementation of a service. Providers typically implement the interface classes, subclasses and subclasses defined in the service itself. Service providers can be installed to extend in the form of implementation of the Java platform, that is, jar files into any of the usual extensions directory. It may also be added by the application class path provider, or made in some other way by a particular platform available. ...... The only requirement is mandatory, the provider must have a class constructor with no arguments, so that they can be instantiated during loading.

By resource directory META-INF/servicesprovider configuration file to identify the service provider is placed in. File names are service type of fully qualified binary name. The binary file contains a list of specific provider class fully qualified names, one per line. Ignore surrounding each name spaces, tabs, and blank lines. The comment character is '#'('\u0023', NUMBER SIGN); ignore all characters following the first comment of characters per line. The file must use UTF-8 encoding.

To 延迟方式查找和实例化提供者, that is to say as needed. Loader maintenance service provider caches so far has been loaded. Each call returns an iterator iterator method, it first generates all the elements of the cache according to Example sequence, then find and instantiate any remaining providers in a delayed manner, adding each one to the cache. You can clear the cache by reload method.

ServiceLoader, like ClassLoader same class file can be loaded, but there are differences when using specific differences are as follows:

1. It is a series of loading ServiceLoader 某种共同特征implementation class, while a universal ClassLoader loader;
2. Require special configuration when loading ServiceLoader, also differ ClassLoader use;
3. ServiceLoader also implements the Iterator interface.

3. Case

Basic services: IService

package com.service;
public  interface IService {
    String sayHello();
    String getScheme();
}

Specific services implementation, here deliberately wrote two

package com.impl;
import com.service.IService;
public  class HDFSService implements IService {
    @Override
    public String sayHello() {
        return "Hello HDFSService";
    }
    @Override
    public String getScheme() {
        return "hdfs";
    }
}

package com.impl;
import com.service.IService;
public  class LocalService  implements IService {
    @Override
    public String sayHello() {
        return "Hello LocalService";
    }
    @Override
    public String getScheme() {
        return "local";
    }
}

配置：META-INF/services/com.service.IService

com.impl.HDFSService
com.impl.LocalService

Test category

package com.test;
import java.util.ServiceLoader;
import com.service.IService;
public  class Test {
    public static void main(String[] args) {
        ServiceLoader<IService> serviceLoader  = ServiceLoader.load(IService.class);
        for(IService service : serviceLoader) {
            System.out.println(service.getScheme()+"="+service.sayHello());
        }
    }
}

result:

hdfs=Hello HDFSService
local=Hello LocalService

ServiceLoader can be seen to define two classes implement according IService find out, to achieve a return of ServiceLoader, and ServiceLoader achieve the Iterable interface, it is possible to traverse all the instances defined in the configuration file by ServiceLoader class.

4.ServiceLoader applications

4.1 Hadoop FileSystem

Hadoop FileSystem is through this mechanism to return to a different FileSystem scheme according to different files.

private static void loadFileSystems() {  
  synchronized(FileSystem.class){  
    if(!FILE_SYSTEMS_LOADED) {  
      ServiceLoader<FileSystem> serviceLoader = ServiceLoader.load(FileSystem.class); 
      for(FileSystem fs : serviceLoader) {  
        SERVICE_FILE_SYSTEMS.put(fs.getScheme(),fs.getClass());  
      } 
      FILE_SYSTEMS_LOADED= true; 
    } 
  } 
}

The corresponding configuration file:

org.apache.hadoop.fs.LocalFileSystem 
org.apache.hadoop.fs.viewfs.ViewFileSystem 
org.apache.hadoop.fs.s3.S3FileSystem 
org.apache.hadoop.fs.s3native.NativeS3FileSystem 
org.apache.hadoop.fs.kfs.KosmosFileSystem 
org.apache.hadoop.fs.ftp.FTPFileSystem 
org.apache.hadoop.fs.HarFileSystem

By testing before the output corresponding to the class and class scheme as follows:

file=class org.apache.hadoop.fs.LocalFileSystem   
viewfs=class org.apache.hadoop.fs.viewfs.ViewFileSystem   
s3=class org.apache.hadoop.fs.s3.S3FileSystem   
s3n=class org.apache.hadoop.fs.s3native.NativeS3FileSystem   
kfs=class org.apache.hadoop.fs.kfs.KosmosFileSystem   
ftp=class org.apache.hadoop.fs.ftp.FTPFileSystem   
har=class org.apache.hadoop.fs.HarFileSystem   
hdfs=class org.apache.hadoop.hdfs.DistributedFileSystem   
hftp=class org.apache.hadoop.hdfs.HftpFileSystem   
hsftp=class org.apache.hadoop.hdfs.HsftpFileSystem   
webhdfs=class org.apache.hadoop.hdfs.web.WebHdfsFileSystem  

It can be seen that all implementations will have to the FileSystem FileSystem class scheme and to cache, then to take the appropriate value from the cache. Therefore, in the future it can be achieved through some similar features ServiceLoader, without relying on third-party frameworks such as Spring.

4.2 sprak achieve

4.3 spring boot

spring boot, there are many places are using a similar configuration, or is used in this way.

Reference: the Spring: What is this Factories exists

4.3.1 SpringBoot Starters starter

Starters can be understood as a starter, which contains a series of applications which can be integrated into dependencies, no matter what frame you want to use technology, a lead startercan be a good make Springand other framework integration, no additional integration work . If you want to use Spring JPAto access the database, just add spring-boot-starter-data-jpastarter depend ready to use.

Spring is how the design of starter

There are many surrounding blog describes how to customize the starter, you will need to add a configuration file spring.factories in the META-INF directory of resources in the last step

SpringBoot startup scans all the resource files spring.factories jar package introduced, the reading of which AutoConfiguration autoconfiguration class starter spring to register class management container.
The mechanism is based on the idea of SPI, its core class SpringFactoriesLoader with similar ServiceLoader

4.4 SPI

spi is the principle.

ServiceLoader is an implementation of SPI, a so-called SPI, i.e. Service Provider Interface, used to achieve some of the services provided to the third party or extended, can be extended to enhance or replace some components of the frame.

SPI after the full name of Service Provider Interface, as service providers, provides an implementation of the service interface, service interface to simultaneously create a file named META-INF jar package / services / directory. The document is the realization of the service implementation class interface. And when the external program module assembly, to find a specific category by the name of the jar package META-INF / services / in the configuration file loading instantiates and complete injection module.

4.5 common-logging

the first to provide facade apache log interface. Only the interface has not been achieved. Specific programs to achieve by the provider, log provider is found by scanning META-INF/services/org.apache.commons.logging.LogFactorythe configuration file, by reading the contents of the log file to find mention industrial and commercial implementation class. As long as we realize the log contains the file, and to develop in the file LogFactoryclass that implements the interface to the plant.

4.6 SLF4J

Logging framework SLF4J chose this path. SLF4J itself API, but may use different implementations (e.g. Logback, Log4J, etc.), SLF4J client only interact with SLF4J the API abstract interface module, it is possible in a different manner is placed on the classpath different implementation module at runtime details of the deal.

4.7 jdbc

jdbc4.0 Previously, developers also need to be based Class.forName("xxx")ways to load the driver, jdbc4 also spi-based mechanism to find the driver provider, you can META-INF/services/java.sql.Driverspecify the implementation class to expose driven provider file.