上文分析類載入機制 機製的文章中我們提高SPI（Service Provider Interface：服務提供者介面）。
我們知道在許多系統裡抽象的各個模組，往往有很多不同的實現方案，比如日誌模組，xml解析模組、jdbc模組的方案等。
在面向的物件的設計裡，我們推薦模組之間基於介面程式設計，模組之間不對實現類進行寫死。爲了實現在模組裝配的時候能不在程式裡動態指明，這就需要一種服務發現機制 機製。在模組化設計中這個機制 機製尤其重要。
SPI就是提供這樣的一個機制 機製：爲某個介面尋找服務實現的機制 機製。它通過查詢classpath路徑下的META-INF/services資料夾下的組態檔，自動載入檔案裡所定義的類（jdbc的jar的classpath對應的META-INF/services/下就有這麼一個組態檔）。
閒話少說，上程式碼：
// 1. 首先 載入sqlite需要的Driver並註冊到DriverManager中
Class.forName("org.sqlite.JDBC");
Connection connection = DriverManager.getConnection("jdbc:sqlite:test.db");
然後，看一下DriverManager.getConnection
@CallerSensitive
public static Connection getConnection(String url)
throws SQLException {
java.util.Properties info = new java.util.Properties();
return (getConnection(url, info, Reflection.getCallerClass()));
}
繼續跟進
// Worker method called by the public getConnection() methods.
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;
if (callerCL == null) {
callerCL = Thread.currentThread().getContextClassLoader();
}
if (url == null) {
throw new SQLException("The url cannot be null", "08001");
}
println("DriverManager.getConnection(\"" + url + "\")");
// 2. ServiceLoader.load載入
ensureDriversInitialized();
// 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());
// 3. 獲得對應的Connection類
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");
}
我們看一下ensureDriversInitialized的實現，SPL的載入在這裏進行的
/*
* Load the initial JDBC drivers by checking the System property
* jdbc.drivers and then use the {@code ServiceLoader} mechanism
*/
private static void ensureDriversInitialized() {
if (driversInitialized) {
return;
}
synchronized (lockForInitDrivers) {
if (driversInitialized) {
return;
}
String drivers;
try {
drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
public String run() {
return System.getProperty(JDBC_DRIVERS_PROPERTY);
}
});
} 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() {
// SPL的載入在這裏進行的
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("")) {
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);
}
}
}
driversInitialized = true;
println("JDBC DriverManager initialized");
}
}
最後通過**Connection con = aDriver.driver.connect(url, info);**獲得Connection範例。
優點
缺點
本文我們對SPI進行了分析，下一篇文章，我們將要對類載入機制 機製的實際應用熱載入進行分析。
https://www.jianshu.com/simple/upgrade
https://www.jianshu.com/p/46b42f7f593c