Java SPI机制分析，linux操作系统和arm架构
org.postgresql
postgresql
42.2.2
com.microsoft.sqlserver
mssql-jdbc
7.0.0.jre8
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() {
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() {
public Void run() {
ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);
Iterator 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);
}
}
}
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
{
Class service;
ClassLoader loader;
Enumeration configs = null;
Iterator pending = null;
String nextName = null;
private LazyIterator(Class 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©) {
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，然后通过类加载器去获取所有类路径下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
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{
/*
-
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;
synchronized(DriverManager.class) {
// synchronize loading of the correct classloader.
if (callerCL == null) {
callerCL = Thread.currentThread().getContextClassLoader();
}
}