目录
前言
分析
前言
【Web】浅浅地聊JDBC java.sql.Driver的SPI后门-CSDN博客
上篇文章我们做到了知其然，知道了JDBC有SPI机制，并且可以利用其Driver后门
这篇文章希望可以做到知其所以然，对JDBC的SPI机制的来源做到心里有数
分析
先是回顾一下JDBC的代码
package com.spi;import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;public class Jdbc_Demo {public static void main(String[] args) throws Exception {// 注册驱动（可以删去）
// Class.forName("java.sql.Driver");// 获取数据库连接对象Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/security","root", "root");// 定义sql语句String sql = "select * from users";// 获取执行sql的对象StatementStatement stmt = con.createStatement();// 执行sqlResultSet res = stmt.executeQuery(sql);// 处理对象while(res.next()) {System.out.println(res.getString("username"));}// 释放资源res.close();stmt.close();con.close();}
}我们可以看到
Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/security","root", "root");
因此JVM会尝试去加载DriverManager类，进而执行DriverManager的静态代码，调用类中的loadInitialDrivers方法（稍微精简了一下代码）
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"); // 也可以通过设置系统属性来加载驱动}});} //...AccessController.doPrivileged(new PrivilegedAction<Void>() {public Void run() {ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);Iterator<Driver> driversIterator = loadedDrivers.iterator();try{while(driversIterator.hasNext()) {driversIterator.next();}}// ....String[] driversList = drivers.split(":");for (String aDriver : driversList) {try {Class.forName(aDriver, true,ClassLoader.getSystemClassLoader());} // ...}
} 重点关注ServiceLoader.load(Driver.class)进行了类加载
public static <S> ServiceLoader<S> load(Class<S> service) {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return ServiceLoader.load(service, cl);
}
跟进ServiceLoader.load(service, cl);
public static <S> ServiceLoader<S> load(Class<S> service,ClassLoader loader){return new ServiceLoader<>(service, loader);}
service是Driver.class，loader是 Thread.currentThread().getContextClassLoader()
这里其实还有双亲委派机制的打破可以讲，但强行塞有点过于臃肿了，略去
跟进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();
}
跟进reload();
public void reload() {providers.clear();lookupIterator = new LazyIterator(service, loader);
}
最后得到的driversIterator就是LazyIterator
OK我们再看下上面代码的这段局部
while(driversIterator.hasNext()) {driversIterator.next();}在这段代码中调用 driversIterator.next()之时，便是调用LazyIterator#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);}}跟进LazyIterator#nextService
private S nextService() {if (!hasNextService())throw new NoSuchElementException();String cn = nextName;nextName = null;Class<?> c = null;try {c = Class.forName(cn, false, loader);} 这段代码的作用是根据存储的类名动态加载一个类(Class.forName)
要注意一点：将 initialize 参数设置为 false，可以实现延迟类的静态初始化，静态初始化块和静态变量的赋值是在类第一次被加载时执行的，如果将 initialize 参数设置为 false，则这些静态操作将被延迟到后续使用该类的过程中才被执行。
类名是哪来的？张口就来吗？我知道你很急，但你先别急，下面就讲。
String cn = nextName;
看到nextName直接就懂了哇，回头看driversIterator.hasNext()，即LazyIterator#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);}}跟进LazyIterator#hasNextService
private boolean hasNextService() {if (nextName != null) {return true;}if (configs == null) {try {String fullName = PREFIX + service.getName();if (loader == null)configs = ClassLoader.getSystemResources(fullName);elseconfigs = 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;}ServiceLoader<S>类有常量属性PREFIX = "META-INF/services/"
service是传入的Driver.class，service.getName()获取Driver接口全类名，拼接得到SPI文件名，给后续读取得到实现类的类名，最后赋值给nextName，再交给LazyIterator#nextService去进行类的动态加载
while ((pending == null) || !pending.hasNext()) {if (!configs.hasMoreElements()) {return false;}pending = parse(service, configs.nextElement());}nextName = pending.next();return true;上面这段话，白话来讲就是，JVM去META-INF/services/下去找Driver接口名的文件，把文件中的内容读出来，也就是我们所要加载的类名，并交由Class.forName来进行动态类的加载。
至此，SPI大成！
