SPI原理-线程上下文类加载器-向日葵屋

SPI原理-线程上下文类加载器

灰太狼 2023-03-03 14:29 26阅读 0赞

最近在看spring源码时，发现了一个方法ClassUtils.getDefaultClassLoader(),先看一下源码

public static ClassLoader getDefaultClassLoader(){
        ClassLoader cl = null;
        try {
            cl = Thread.currentThread().getContextClassLoader();
        }catch (Throwable ex){
            // Cannot access thread context ClassLoader - falling back...
        }
        if(cl == null){
            // No thread context class loader -> use class loader of this class.
            cl = ClassUtils.class.getClassLoader();
            if(cl == null){
                // getClassLoader() returning null indicates the bootstrap ClassLoader
                try {
                    cl = ClassLoader.getSystemClassLoader();
                }catch (Throwable ex){
                    // Cannot access system ClassLoader - oh well, maybe the caller can live with null...
                }
            }
        }
        return cl;
    }

看到这里我感觉有点儿好奇，为什么这里要先从Thread.currentThread().getContextClassLoader()这样来获取类加载器呢，而不是直接使用下面的方式来获取类加载器。Thread.currentThread().getContextClassLoader()获取的是线程上下文的类加载器。这个也是spi的原理。

 spi打破了类加载器的双亲委派模型，底层就是使用的是线程上下文类加载器。
 例如jdbc就是这样，我们知道接口是jdk提供的，具体实现由厂商来实现，那么父类加载器如何加载应用类加载的类呢。看一下源码。
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);
                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("")) {
            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);
            }
        }
    }

核心方法就是ServiceLoader.load(Driver.class);然后点进去看具体的实现

public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }