【Java实战】源码解析Java SPI(Service Provider Interface )机制原理
一、背景知识
在阅读开源框架源码时,发现许多框架都支持SPI(Service Provider Interface ),前面有篇文章JDBC对Driver的加载时应用了SPI,参考【Hibernate实战】源码解析Hibernate参数绑定及PreparedStatement防SQL注入原理,于是借着JDBC对Driver的加载实现,分析下SPI机制。
二、什么是SPI
Service Provider Interface (SPI) is an API intended to be implemented or extended by a third party. It can be used to enable framework extension and replaceable components.
简单翻译下:
A service provider interface (SPI) is the set of public interfaces and abstract classes that a service defines. The SPI defines the classes and methods available to your application. A service provider implements the SPI. An application with extensible services will allow you, vendors, and perhaps even customers to add service providers without modifying the original application.
三、SPI原理
1、JDBC示例程序
public static void JDBCExample(){ try { //Class.forName("com.mysql.jdbc.Driver"); Connection connection = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/hhl?useServerPrepStmts=true&cachePrepStmts=true&prepStmtCacheSize=25&prepStmtCacheSqlLimit=2048&characterEncoding=utf8&useSSL=false", "root", "123456"); PreparedStatement preparedStatement = connection.prepareStatement("SELECT * FROM product p WHERE p.productName=?"); preparedStatement.setString(1,"Mango"); ResultSet resultSet = preparedStatement.executeQuery(); while (resultSet.next()){ System.out.println(resultSet.getString(1)); } resultSet.close(); preparedStatement.close(); connection.close(); } catch (Exception e) { e.printStackTrace(); } }
/** * Load the initial JDBC drivers by checking the System property * jdbc.properties and then use the {@code ServiceLoader} mechanism */ static { loadInitialDrivers(); println("JDBC DriverManager initialized"); }
private static void loadInitialDrivers() { String drivers; try { drivers = AccessController.doPrivileged(new PrivilegedAction<String>() { public String run() { return System.getProperty("jdbc.drivers"); } }); } catch (Exception ex) { drivers = null; } // If the driver is packaged as a Service Provider, load it. // Get all the drivers through the classloader // exposed as a java.sql.Driver.class service. // ServiceLoader.load() replaces the sun.misc.Providers() AccessController.doPrivileged(new PrivilegedAction<Void>() { public Void run() { ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class); Iterator<Driver> driversIterator = loadedDrivers.iterator(); /* Load these drivers, so that they can be instantiated. * It may be the case that the driver class may not be there * i.e. there may be a packaged driver with the service class * as implementation of java.sql.Driver but the actual class * may be missing. In that case a java.util.ServiceConfigurationError * will be thrown at runtime by the VM trying to locate * and load the service. * * Adding a try catch block to catch those runtime errors * if driver not available in classpath but it's * packaged as service and that service is there in classpath. */ try{ while(driversIterator.hasNext()) { driversIterator.next(); } } catch(Throwable t) { // Do nothing } return null; } }); println("DriverManager.initialize: jdbc.drivers = " + drivers); if (drivers == null || drivers.equals("")) { return; } String[] driversList = drivers.split(":"); println("number of Drivers:" + driversList.length); for (String aDriver : driversList) { try { println("DriverManager.Initialize: loading " + aDriver); Class.forName(aDriver, true, ClassLoader.getSystemClassLoader()); } catch (Exception ex) { println("DriverManager.Initialize: load failed: " + ex); } } }
ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class); 2 Iterator<Driver> driversIterator = loadedDrivers.iterator(); 3 try{ while(driversIterator.hasNext()) { 4 driversIterator.next(); 5 } } catch(Throwable t) { // Do nothing }
2、逐行看下 java.util.ServiceLoader.java
public static <S> ServiceLoader<S> load(Class<S> service) { ClassLoader cl = Thread.currentThread().getContextClassLoader(); return ServiceLoader.load(service, cl); }
public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader) { return new ServiceLoader<>(service, loader); }
private ServiceLoader(Class<S> svc, ClassLoader cl) { service = Objects.requireNonNull(svc, "Service interface cannot be null"); loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl; acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null; reload(); }
public void reload() { providers.clear(); lookupIterator = new LazyIterator(service, loader); }
// Cached providers, in instantiation order private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
// The current lazy-lookup iterator private LazyIterator lookupIterator;
// Private inner class implementing fully-lazy provider lookup // private class LazyIterator implements Iterator<S> { Class<S> service; ClassLoader loader; Enumeration<URL> configs = null; Iterator<String> pending = null; String nextName = null; private LazyIterator(Class<S> service, ClassLoader loader) { this.service = service; this.loader = loader; } private boolean hasNextService() { if (nextName != null) { return true; } if (configs == null) { try { String fullName = PREFIX + service.getName(); if (loader == null) configs = ClassLoader.getSystemResources(fullName); else configs = loader.getResources(fullName); } catch (IOException x) { fail(service, "Error locating configuration files", x); } } while ((pending == null) || !pending.hasNext()) { if (!configs.hasMoreElements()) { return false; } pending = parse(service, configs.nextElement()); } nextName = pending.next(); return true; } private S nextService() { if (!hasNextService()) throw new NoSuchElementException(); String cn = nextName; nextName = null; Class<?> c = null; try { c = Class.forName(cn, false, loader); } catch (ClassNotFoundException x) { fail(service, "Provider " + cn + " not found"); } if (!service.isAssignableFrom(c)) { fail(service, "Provider " + cn + " not a subtype"); } try { S p = service.cast(c.newInstance()); providers.put(cn, p); return p; } catch (Throwable x) { fail(service, "Provider " + cn + " could not be instantiated", x); } throw new Error(); // This cannot happen } public boolean hasNext() { if (acc == null) { return hasNextService(); } else { PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() { public Boolean run() { return hasNextService(); } }; return AccessController.doPrivileged(action, acc); } } public S next() { if (acc == null) { return nextService(); } else { PrivilegedAction<S> action = new PrivilegedAction<S>() { public S run() { return nextService(); } }; return AccessController.doPrivileged(action, acc); } } public void remove() { throw new UnsupportedOperationException(); } }
创建改对象的原因就是为了实现延迟服务提供者查找。延迟到什么时候,继续看代码。
3、loadedDrivers.iterator()
public Iterator<S> iterator() { return new Iterator<S>() { Iterator<Map.Entry<String,S>> knownProviders = providers.entrySet().iterator(); public boolean hasNext() { if (knownProviders.hasNext()) return true; return lookupIterator.hasNext(); } public S next() { if (knownProviders.hasNext()) return knownProviders.next().getValue(); return lookupIterator.next(); } public void remove() { throw new UnsupportedOperationException(); } }; }
创建了一个内部类Iterator,用于操作缓存providers和延迟加载类lookupIterator
4、driversIterator.hasNext()
public boolean hasNext() { if (knownProviders.hasNext()) return true; return lookupIterator.hasNext(); }
用到了LazyIterator中的hasNext()
public boolean hasNext() { if (acc == null) { return hasNextService(); } else { PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() { public Boolean run() { return hasNextService(); } }; return AccessController.doPrivileged(action, acc); } }
private boolean hasNextService() { if (nextName != null) { return true; } if (configs == null) { try { String fullName = PREFIX + service.getName(); if (loader == null) configs = ClassLoader.getSystemResources(fullName); else configs = loader.getResources(fullName); } catch (IOException x) { fail(service, "Error locating configuration files", x); } } while ((pending == null) || !pending.hasNext()) { if (!configs.hasMoreElements()) { return false; } pending = parse(service, configs.nextElement()); } nextName = pending.next(); return true; }
private static final String PREFIX = "META-INF/services/";
com.mysql.jdbc.Driver com.mysql.fabric.jdbc.FabricMySQLDriver
private Iterator<String> parse(Class<?> service, URL u) throws ServiceConfigurationError { InputStream in = null; BufferedReader r = null; ArrayList<String> names = new ArrayList<>(); try { in = u.openStream(); r = new BufferedReader(new InputStreamReader(in, "utf-8")); int lc = 1; while ((lc = parseLine(service, u, r, lc, names)) >= 0); } catch (IOException x) { fail(service, "Error reading configuration file", x); } finally { try { if (r != null) r.close(); if (in != null) in.close(); } catch (IOException y) { fail(service, "Error closing configuration file", y); } } return names.iterator(); }
5、driversIterator.next()
接着看Next
public S next() { if (knownProviders.hasNext()) return knownProviders.next().getValue(); return lookupIterator.next(); }
起初走lookupIterator.next()
public S next() { if (acc == null) { return nextService(); } else { PrivilegedAction<S> action = new PrivilegedAction<S>() { public S run() { return nextService(); } }; return AccessController.doPrivileged(action, acc); } }
直接走nextService
private S nextService() { if (!hasNextService()) throw new NoSuchElementException(); String cn = nextName; nextName = null; Class<?> c = null; try { c = Class.forName(cn, false, loader); } catch (ClassNotFoundException x) { fail(service, "Provider " + cn + " not found"); } if (!service.isAssignableFrom(c)) { fail(service, "Provider " + cn + " not a subtype"); } try { S p = service.cast(c.newInstance()); providers.put(cn, p); return p; } catch (Throwable x) { fail(service, "Provider " + cn + " could not be instantiated", x); } throw new Error(); // This cannot happen }
这里会加载并初始化获取到的驱动,例如com.mysql.jdbc.Driver,这里还是需要Class.forName。采用
S p = service.cast(c.newInstance()); providers.put(cn, p);
初始化驱动,因此驱动需要有一个默认的构造函数。
至此,利用ServiceLoader加载并初始化驱动的操作就完成了。那么那么多驱动,要选择哪个驱动呢,就是根据url确定
jdbc:mysql://127.0.0.1:3306/hhl
如上url就会选择com.mysql.jdbc.Driver进行连接操作,其判断由驱动自己去做,由驱动中的acceptsURL及parseURL判断驱动支持的url,具体可看代码com.mysql.jdbc.NonRegisteringDriver.java
总结:SPI的机制就是在不修改原有程序的基础上实现扩展
当服务的提供者,提供了服务接口(java.sql.Driver)的一种实现之后,在jar包的META-INF/services/目录里同时创建一个以服务接口命名的文件。该文件里就是实现该服务接口的具体实现类。而当外部程序装配这个模块的时候,就能通过该jar包META-INF/services/里的配置文件找到具体的实现类名,并装载实例化,完成模块的注入。 基于这样一个约定就能很好的找到服务接口的实现类,而不需要再代码里指定。
【Java实战】源码解析Java SPI(Service Provider Interface )机制原理的更多相关文章
- java的spi(Service Provider Interface)机制及源码(java8)
1.什么是java的spi spi 全称为 service provider interface 即 服务提供接口,用来作为服务的扩展发现.在运行时动态添加接口的实现,是对接口的实现类的创建管理. 2 ...
- SPI(Service Provider Interface)机制
JAVA SPI 约定如下:当服务的提供者提供了服务接口的一种实现之后,在jar包的META-INF/services/ 目录中同时创建一个以服务接口命名的文件,该文件中的内容就是实现该服务接口的具体 ...
- 【转】Java HashMap 源码解析(好文章)
.fluid-width-video-wrapper { width: 100%; position: relative; padding: 0; } .fluid-width-video-wra ...
- Java集合类源码解析:Vector
[学习笔记]转载 Java集合类源码解析:Vector 引言 之前的文章我们学习了一个集合类 ArrayList,今天讲它的一个兄弟 Vector.为什么说是它兄弟呢?因为从容器的构造来说,Vec ...
- Java——LinkedHashMap源码解析
以下针对JDK 1.8版本中的LinkedHashMap进行分析. 对于HashMap的源码解析,可阅读Java--HashMap源码解析 概述 哈希表和链表基于Map接口的实现,其具有可预测的迭 ...
- Dubbo源码解析之SPI(一):扩展类的加载过程
Dubbo是一款开源的.高性能且轻量级的Java RPC框架,它提供了三大核心能力:面向接口的远程方法调用.智能容错和负载均衡,以及服务自动注册和发现. Dubbo最早是阿里公司内部的RPC框架,于 ...
- Volley源码解析(三) 有缓存机制的情况走缓存请求的源码分析
Volley源码解析(三) 有缓存机制的情况走缓存请求的源码分析 Volley之所以高效好用,一个在于请求重试策略,一个就在于请求结果缓存. 通过上一篇文章http://www.cnblogs.com ...
- [源码解析] TensorFlow 分布式环境(8) --- 通信机制
[源码解析] TensorFlow 分布式环境(8) --- 通信机制 目录 [源码解析] TensorFlow 分布式环境(8) --- 通信机制 1. 机制 1.1 消息标识符 1.1.1 定义 ...
- JAVA SPI(Service Provider Interface)原理、设计及源码解析(其一)
背景 团队内部轮流技术分享,其他人都是分享源码,我每次都是设计和架构,感觉自己太特立独行.这次我要合群点,分享点源码. 概念 Service Provider Interface:服务提供方接口.是一 ...
随机推荐
- 20145312《Java程序设计》课程总结
20145312<Java程序设计>课程总结 每周读书笔记链接汇总 20145312<Java程序设计>第一周学习总结 20145312<Java程序设计>第二周学 ...
- 20145230熊佳炜《网络对抗》实验九:web安全基础实践
20145230熊佳炜<网络对抗>实验九:web安全基础实践 webgoat webgoat的中文是代罪羔羊的意思,而它是一个有很多漏洞的web应用程序,我们可以利用它来研究关于web应用 ...
- Linux kernel 编译问题记录【转】
本文转载自:http://sunyongfeng.com/201701/programmer/linux/kernel_compile_fail.html 编译内核出现以下错误 Can't use ' ...
- Spring Container的扩展点
转自: http://blog.csdn.net/kkdelta/article/details/5488430 Spring在解析完配置文件后,会调用一些callback方法,使用Spring的开发 ...
- 解题报告:51nod 加农炮
2017-10-07 16:15:16 writer:pprp 题目来源: Codility 基准时间限制:1 秒 空间限制:131072 KB 分值: 40 难度:4级算法题 一个长度为M的正整 ...
- from: can't read /var/mail/xxx 解决方法
在执行一个发包脚本的时候,遇到了如下问题: from: can't read /var/mail/scapy.all 原因:脚本是没有问题的,但它并不是可以被python执行的可执行文件. 解决方法: ...
- Hadoop 常用指令
1. 察看hdfs文件系统运行情况 bin/hdfs dfsadmin -report 2. 为了方便执行 HDFS 的操作指令,我们可以将需要的 Hadoop 路径写入环境变量中,便于直接执行命令. ...
- Redis可以做哪些事儿?
Redis可以作为数据库,提供高速缓存,消息队列等功能,这里介绍Redis可以做的其中两件事: 1.提供缓存功能,作为缓存服务器; 2.轻量级的消息队列(MQ)进行使用. /// <summar ...
- ios 下拉刷新开源框架 MJRefresh
gitHub 下载框架 搜索MJExampleViewController.h 下拉刷新 MJTableViewController 上拉刷新 MJTableViewController Collec ...
- 【Python】 \uxxxx转中文
背景 写Python接口自动化过程中,使用到邮件发送测试结果详情,邮件呈现出来的内容为 \uxxxx ,不是中文 接收到的邮件内容: 成功: 110 失败: 1 失败的用例如下 : [(u'\u752 ...