JDBC SPI 类加载机制
https://segmentfault.com/a/1190000017517197?utm_source=tag-newest
一句话总结：
1 自动加载：DriverManager的静态代码块执行的时刻，使用该时刻当前线程类加载器加载java.sql.Driver文件，并用当前线程类加载器加载及、实例化、registerDriver
2 没有自动加载的，手动forName，传入一个类加载器及是否初始化；或loadClass➕实例化（load/find class 与 forname 在static代码块加载的不同 （二）非系统类jdbc）
3 getConncetion时，校验调用getConnection的类所在类加载器与driver实现所在类加载器（校验存放的driver是否属于调用者的Classloader），JDBC注册原理与自定义类加载器解决com.cloudera.hive.jdbc41.HS2Driver的加载【重点】 这个连接通过伪造一个中间人骗过校验
SPI概述
SPI全称为(Service Provider Interface) ，是JDK内置的一种服务提供发现机制；主要被框架的开发人员使用，比如java.sql.Driver接口，数据库厂商实现此接口即可，当然要想让系统知道具体实现类的存在，还需要使用固定的存放规则，需要在classpath下的META-INF/services/目录里创建一个以服务接口命名的文件，这个文件里的内容就是这个接口的具体的实现类；下面以JDBC为实例来进行具体的分析。
JDBC驱动
1.准备驱动包
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.47</version>
</dependency>
<dependency>
<groupId>org.postgresql</groupId>
<artifactId>postgresql</artifactId>
<version>42.2.2</version>
</dependency>
<dependency>
<groupId>com.microsoft.sqlserver</groupId>
<artifactId>mssql-jdbc</artifactId>
<version>7.0.0.jre8</version>
</dependency>分别准备了mysql，postgresql和sqlserver，可以打开jar，发现每个jar包的META-INF/services/都存在一个java.sql.Driver文件，文件里面存在一个或多个类名，比如mysql：
com.mysql.jdbc.Driver
com.mysql.fabric.jdbc.FabricMySQLDriver提供的每个驱动类占据一行，解析的时候会按行读取，具体使用哪个会根据url来决定；
2.简单实例
String url = "jdbc:mysql://localhost:3306/db3";
String username = "root";
String password = "root";
String sql = "update travelrecord set name=\'bbb\' where id=1";
Connection con = DriverManager.getConnection(url, username, password);类路径下存在多个驱动包，具体在使用DriverManager.getConnection应该使用哪个驱动类会解析url来识别，不同的数据库有不同的url前缀；
3.驱动类加载分析
具体META-INF/services/下的驱动类是什么时候加载的，DriverManager有一个静态代码块：
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);【会遍历DriverManage静态代码块执行时，当前线程类加载器下找】
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();【重要，在这一句forname和实例化】
}
} catch(Throwable t) {
// Do nothing
}
return null;
}
});
println("DriverManager.initialize: jdbc.drivers = " + drivers);
【第二种方式】
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);
}
}
}在加载DriverManager类的时候会执行loadInitialDrivers方法，方法内通过了两种加载驱动类的方式，分别是：使用系统变量方式和ServiceLoader加载方式；系统变量方式其实就是在变量jdbc.drivers中配置好驱动类，然后使用Class.forName进行加载；下面重点看一下ServiceLoader方式，此处调用了load方法但是并没有真正去加载驱动类，而是返回了一个LazyIterator，后面的代码就是循环变量迭代器：
private static final String PREFIX = "META-INF/services/";
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(); service：java.sql.Driver
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
configs = loader.getResources(fullName);【重要，遍历META-INF/servises/java.sql.Driver资源文件】
} 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); 【重要，使用传入的当前线程类加载器加载】
} 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
}
......
}类中指定了一个静态常量PREFIX = “META-INF/services/”，然后和java.sql.Driver拼接组成了fullName，然后通过类加载器(classloader getresource jar包资源冲突情况，父亲为大，兄弟之间，谁先加载谁牛逼（二）)去获取所有类路径（当前类加载）下java.sql.Driver文件，获取之后存放在configs中，里面的每个元素对应一个文件，每个文件中可能会存在多个驱动类，所以使用pending用来存放每个文件中的驱动信息，获取驱动信息之后在nextService中使用Class.forName加载类信息，并且指定不进行初始化；同时在下面使用newInstance 触发各驱动的静态代码块（load/find class 与 forname 在static代码块加载的不同）对驱动类进行了实例化操作；每个驱动类中都提供了一个静态注册代码块，比如mysql：
static {
try {
java.sql.DriverManager.registerDriver(new Driver());
} catch (SQLException E) {
throw new RuntimeException("Can't register driver!");
}
}这里又实例化了一个驱动类，同时注册到DriverManager；接下来就是调用DriverManager的getConnection方法，代码如下：
private static Connection getConnection(
String url, java.util.Properties info, Class<?> caller) throws SQLException {
/*
* When callerCl is null, we should check the application's
* (which is invoking this class indirectly)
* classloader, so that the JDBC driver class outside rt.jar
* can be loaded from here.
*/
ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;【调用方的类所在类加载器】
synchronized(DriverManager.class) {
// synchronize loading of the correct classloader.
if (callerCL == null) {
callerCL = Thread.currentThread().getContextClassLoader();
}
}
if(url == null) {
throw new SQLException("The url cannot be null", "08001");
}
println("DriverManager.getConnection(\"" + url + "\")");
// Walk through the loaded registeredDrivers attempting to make a connection.
// Remember the first exception that gets raised so we can reraise it.
SQLException reason = null;
for(DriverInfo aDriver : registeredDrivers) {
// If the caller does not have permission to load the driver then
// skip it.
if(isDriverAllowed(aDriver.driver, callerCL)) {（JDBC注册原理与自定义类加载器解决com.cloudera.hive.jdbc41.HS2Driver的加载【重点】中用一个代理中间人骗过了）
try {
println(" trying " + aDriver.driver.getClass().getName());
Connection con = aDriver.driver.connect(url, info);
if (con != null) {
// Success!
println("getConnection returning " + aDriver.driver.getClass().getName());
return (con);
}
} catch (SQLException ex) {
if (reason == null) {
reason = ex;
}
}
} else {
println(" skipping: " + aDriver.getClass().getName());
}
}
// if we got here nobody could connect.
if (reason != null) {
println("getConnection failed: " + reason);
throw reason;
}
println("getConnection: no suitable driver found for "+ url);
throw new SQLException("No suitable driver found for "+ url, "08001");
}此方法主要是遍历之前注册的DriverInfo，拿着url信息去每个驱动类中建立连接，当然每个驱动类中都会进行url匹配校验，成功之后返回Connection，如果中途有失败的连接并不影响尝试新的驱动连接，遍历完之后还是无法获取连接，则抛出异常；
