Java SPI机制分析(1)，源码解读及如何保证线程安全
}
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
}
......
}
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 {
/*
* 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();
}
}
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)) {
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；
4.扩展
如果想扩展新的驱动类也很简单，只需要在类路径下创建META-INF/services/文件夹，同时在里面创建java.sql.Driver文件，在文件中写入具体的驱动类名称，当然此类需要继承java.sql.Driver接口类；例如实例中提供的TestDriver。
**序列化实战**
1.准备接口类
public interface Serialization {
/**
* 序列化
*
* @param obj
* @return
*/
public byte[] serialize(Object obj) throws Exception;
/**
* 反序列化
*
* @param param
* @param clazz
* @return
* @throws Exception
*/
public T deserialize(byte[] param, Class clazz) throws Exception;
/**
* 序列化名称
*
* @return
*/
public String getName();
}
2.准备实现类
分别准备JsonSerialization和ProtobufSerialization
3.接口文件
在META-INF/services/目录下创建文件com.spi.serializer.Serialization，内容如下：
com.spi.serializer.JsonSerialization
com.spi.serializer.ProtobufSerialization
最后
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m.spi.serializer.ProtobufSerialization
最后
由于篇幅限制，小编在此截出几张知识讲解的图解，有需要的程序猿（媛）可以点赞后戳这里免费领取全部资料获取哦
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