Java类加载器与双亲委派机制和线程上下文类加载器专项解读分析

public static void main(String[] args) throws ClassNotFoundException {
        Class<?> aClass = Class.forName("com.mycompany.load.F");
        System.out.println(aClass.getClassLoader()); // AppClassLoader  ExtClassLoader
    }

package com.mycompany.load;
import sun.misc.Launcher;
public class Load6 {
    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
        ClassLoader classLoader = new Launcher().getClassLoader(); //1 new一个新的类加载器
        System.out.println(classLoader);
        /*
        这是因为 1处获取的应用类加载器a和jvm用来加载Load6.class对象的应用类加载器b不是同一个实例，
        那么构成这两个类的run-time package也就是不同的。所以即使它们的二进制名字相同，
        但是由a定义的Load6类所创建的对象显然不能转化为由b定义的Load6类的实例。
        这种情况下jvm就会抛出ClassCastException
        * */
        Class<?> aClass = classLoader.loadClass("com.mycompany.load.Load6");
        Load6 load6  = (Load6)aClass.newInstance(); //2
        //Exception in thread "main" java.lang.ClassCastException: com.mycompany.load.Load6 cannot be cast to com.mycompany.load.Load6
    }
}

protected Class<?> loadClass(String name, boolean resolve)
            throws ClassNotFoundException {
        synchronized (getClassLoadingLock(name)) {
            // 1. 检查该类是否已经加载
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        // 2. 有上级的话，委派上级 loadClass
                        c = parent.loadClass(name, false);
                    } else {
                        // 3. 如果没有上级了（ExtClassLoader），则委派
                        BootstrapClassLoader
                                c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                }
                if (c == null) {
                    long t1 = System.nanoTime();
                    // 4. 每一层找不到，调用 findClass 方法（每个类加载器自己扩展）来加载
                    c = findClass(name);
                    // 5. 记录耗时
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

System.out.println(DriverManager.class.getClassLoader());

public class DriverManager {
    // 注册驱动的集合
    private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers
        = new CopyOnWriteArrayList<>();
    // 初始化驱动
    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()
        // 1、使用 ServiceLoader 机制加载驱动，即 SPI
        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);
        // 2、使用 jdbc.drivers 定义的驱动名加载驱动
        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);
                // 这里的 ClassLoader.getSystemClassLoader() 就是应用程序类加载器
                Class.forName(aDriver, true,
                        ClassLoader.getSystemClassLoader());
            } catch (Exception ex) {
                println("DriverManager.Initialize: load failed: " + ex);
            }
        }
    }

ServiceLoader<接口类型> allImpls = ServiceLoader.load(接口类型.class);
Iterator<接口类型> iter = allImpls.iterator();
while(iter.hasNext()) {
    iter.next();
}

ServiceLoader<Driver> loader = ServiceLoader.load(Driver.class);
Iterator<Driver> iterator = loader.iterator();
while (iterator.hasNext()){
   Driver dirver = iterator.next();
   System.out.println(dirver.getClass()+", 类加载器："+dirver.getClass().getClassLoader());
}
System.out.println("当前线程上线文类加载器："+Thread.currentThread().getContextClassLoader());
System.out.println("ServiceLoader类加载器："+loader.getClass().getClassLoader());

public static <S> ServiceLoader<S> load(Class<S> service) {
    // 获取线程上下文类加载器
    ClassLoader cl = Thread.currentThread().getContextClassLoader();
    return ServiceLoader.load(service, cl);
}

        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
        }

public class Load7 {
    public static void main(String[] args) throws Exception {
        MyClassLoader classLoader = new MyClassLoader();
        Class<?> c1 = classLoader.loadClass("TestServiceImpl");
        Class<?> c2 = classLoader.loadClass("TestServiceImpl");
        System.out.println(c1 == c2);//true
        MyClassLoader classLoader2 = new MyClassLoader();
        Class<?> c3 = classLoader2.loadClass("TestServiceImpl");
        //虽然相同类名，但不是同一个类加载器加载的
        System.out.println(c1 == c3);//false
        c1.newInstance();
    }
}
class MyClassLoader extends ClassLoader {
    @Override // name 就是类名称
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        String path = "D:\\myclasspath\\" + name + ".class";
        try {
            ByteArrayOutputStream os = new ByteArrayOutputStream();
            Files.copy(Paths.get(path), os);
            // 得到字节数组
            byte[] bytes = os.toByteArray();
            // byte[] -> *.class
            return defineClass(name, bytes, 0, bytes.length);
        } catch (IOException e) {
            e.printStackTrace();
            throw new ClassNotFoundException("类文件未找到", e);
        }
    }
}