一篇文章吃透：为什么加载数据库驱动要用Class.forName()

    @CallerSensitive
    public static Class forName(String className)
                throws ClassNotFoundException {
        Class caller = Reflection.getCallerClass();
        return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
    }

    /**
     * Returns the {@code Class} object associated with the class or
     * interface with the given string name, using the given class loader.
     * Given the fully qualified name for a class or interface (in the same
     * format returned by {@code getName}) this method attempts to
     * locate, load, and link the class or interface.  The specified class
     * loader is used to load the class or interface.  If the parameter
     * {@code loader} is null, the class is loaded through the bootstrap
     * class loader.  The class is initialized only if the
     * {@code initialize} parameter is {@code true} and if it has
     * not been initialized earlier.
     *
     *  If {@code name} denotes a primitive type or void, an attempt
     * will be made to locate a user-defined class in the unnamed package whose
     * name is {@code name}. Therefore, this method cannot be used to
     * obtain any of the {@code Class} objects representing primitive
     * types or void.
     *
     *  If {@code name} denotes an array class, the component type of
     * the array class is loaded but not initialized.
     *
     *  For example, in an instance method the expression:
     *
     * 
     *  {@code Class.forName("Foo")}
     * 
     *
     * is equivalent to:
     *
     * 
     *  {@code Class.forName("Foo", true, this.getClass().getClassLoader())}
     * 
     *
     * Note that this method throws errors related to loading, linking or
     * initializing as specified in Sections 12.2, 12.3 and 12.4 of The
     * Java Language Specification.
     * Note that this method does not check whether the requested class
     * is accessible to its caller.
     *
     *  If the {@code loader} is {@code null}, and a security
     * manager is present, and the caller's class loader is not null, then this
     * method calls the security manager's {@code checkPermission} method
     * with a {@code RuntimePermission("getClassLoader")} permission to
     * ensure it's ok to access the bootstrap class loader.
     *
     * @param name       fully qualified name of the desired class
     * @param initialize if {@code true} the class will be initialized.
     *                   See Section 12.4 of The Java Language Specification.
     * @param loader     class loader from which the class must be loaded
     * @return           class object representing the desired class
     *
     * @exception LinkageError if the linkage fails
     * @exception ExceptionInInitializerError if the initialization provoked
     *            by this method fails
     * @exception ClassNotFoundException if the class cannot be located by
     *            the specified class loader
     *
     * @see       java.lang.Class#forName(String)
     * @see       java.lang.ClassLoader
     * @since     1.2
     */
    @CallerSensitive
    public static Class forName(String name, boolean initialize,
                                   ClassLoader loader)
        throws ClassNotFoundException
    {
        Class caller = null;
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            // Reflective call to get caller class is only needed if a security manager
            // is present.  Avoid the overhead of making this call otherwise.
            caller = Reflection.getCallerClass();
            if (sun.misc.VM.isSystemDomainLoader(loader)) {
                ClassLoader ccl = ClassLoader.getClassLoader(caller);
                if (!sun.misc.VM.isSystemDomainLoader(ccl)) {
                    sm.checkPermission(
                        SecurityConstants.GET_CLASSLOADER_PERMISSION);
                }
            }
        }
        return forName0(name, initialize, loader, caller);
    }

 /**
     * Creates a new instance of the class represented by this {@code Class}
     * object.  The class is instantiated as if by a {@code new}
     * expression with an empty argument list.  The class is initialized if it
     * has not already been initialized.
     *
     * Note that this method propagates any exception thrown by the
     * nullary constructor, including a checked exception.  Use of
     * this method effectively bypasses the compile-time exception
     * checking that would otherwise be performed by the compiler.
     * The {@link
     * java.lang.reflect.Constructor#newInstance(java.lang.Object...)
     * Constructor.newInstance} method avoids this problem by wrapping
     * any exception thrown by the constructor in a (checked) {@link
     * java.lang.reflect.InvocationTargetException}.
     *
     * @return  a newly allocated instance of the class represented by this
     *          object.
     * @throws  IllegalAccessException  if the class or its nullary
     *          constructor is not accessible.
     * @throws  InstantiationException
     *          if this {@code Class} represents an abstract class,
     *          an interface, an array class, a primitive type, or void;
     *          or if the class has no nullary constructor;
     *          or if the instantiation fails for some other reason.
     * @throws  ExceptionInInitializerError if the initialization
     *          provoked by this method fails.
     * @throws  SecurityException
     *          If a security manager, s, is present and
     *          the caller's class loader is not the same as or an
     *          ancestor of the class loader for the current class and
     *          invocation of {@link SecurityManager#checkPackageAccess
     *          s.checkPackageAccess()} denies access to the package
     *          of this class.
     */
    @CallerSensitive
    public T newInstance()
        throws InstantiationException, IllegalAccessException
    {
        if (System.getSecurityManager() != null) {
            checkMemberAccess(Member.PUBLIC, Reflection.getCallerClass(), false);
        }

        // NOTE: the following code may not be strictly correct under
        // the current Java memory model.

        // Constructor lookup
        if (cachedConstructor == null) {
            if (this == Class.class) {
                throw new IllegalAccessException(
                    "Can not call newInstance() on the Class for java.lang.Class"
                );
            }
            try {
                Class[] empty = {};
                final Constructor c = getConstructor0(empty, Member.DECLARED);
                // Disable accessibility checks on the constructor
                // since we have to do the security check here anyway
                // (the stack depth is wrong for the Constructor's
                // security check to work)
                java.security.AccessController.doPrivileged(
                    new java.security.PrivilegedAction() {
                        public Void run() {
                                c.setAccessible(true);
                                return null;
                            }
                        });
                cachedConstructor = c;
            } catch (NoSuchMethodException e) {
                throw (InstantiationException)
                    new InstantiationException(getName()).initCause(e);
            }
        }
        Constructor tmpConstructor = cachedConstructor;
        // Security check (same as in java.lang.reflect.Constructor)
        int modifiers = tmpConstructor.getModifiers();
        if (!Reflection.quickCheckMemberAccess(this, modifiers)) {
            Class caller = Reflection.getCallerClass();
            if (newInstanceCallerCache != caller) {
                Reflection.ensureMemberAccess(caller, this, null, modifiers);
                newInstanceCallerCache = caller;
            }
        }
        // Run constructor
        try {
            return tmpConstructor.newInstance((Object[])null);
        } catch (InvocationTargetException e) {
            Unsafe.getUnsafe().throwException(e.getTargetException());
            // Not reached
            return null;
        }
    }

/**
 * 加载测试demo类
 * 
 * @author: 明明如月 liuwangyanghdu@163.com
 * @date: 2019-04-09 12:43
 */
public class LoadTestClass {

    static {
        System.out.println("静态代码块");
    }

    public LoadTestClass() {
        System.out.println("构造方法");
    }

   public static void test() {
        System.out.println("这是静态方法");
    }
}

/**
 * 加载测试类
 *
 * @author: 明明如月 liuwangyanghdu@163.com
 * @date: 2019-04-09 12:42
 */
public class LoadTest {
 @Test
    public void test(){
        LoadTestClass.test();
    }

   @Test
    public void forName() throws ClassNotFoundException {
        Class aClass = Class.forName("com.chujianyun.common.clazz.LoadTestClass");
        System.out.println(aClass);
    }

    @Test
    public void newTest() {
        new LoadTestClass();
    }

    @Test
    public void loader() throws ClassNotFoundException {
        Class aClass = ClassLoader.getSystemClassLoader().loadClass("com.chujianyun.common.clazz.LoadTestClass");
        System.out.println(aClass);
    }

    @Test
    public void loaderNewInstance() throws ClassNotFoundException, IllegalAccessException, InstantiationException {
        Class aClass = ClassLoader.getSystemClassLoader().loadClass("com.chujianyun.common.clazz.LoadTestClass");
        System.out.println(aClass);
        Object result = aClass.newInstance();
        System.out.println(result instanceof LoadTestClass);
    }
}

String url = "jdbc:mysql://localhost:3306/test";
Connection conn = DriverManager.getConnection(url,username,password);

package com.mysql.cj.jdbc;

import java.sql.DriverManager;
import java.sql.SQLException;

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    public Driver() throws SQLException {
    }

    static {
        try {
            DriverManager.registerDriver(new Driver());
        } catch (SQLException var1) {
            throw new RuntimeException("Can't register driver!");
        }
    }
}

 public static synchronized void registerDriver(java.sql.Driver driver)
        throws SQLException {

        registerDriver(driver, null);
    }

/**
 * Load the initial JDBC drivers by checking the System property
 * jdbc.properties and then use the {@code ServiceLoader} mechanism
 */
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);
            }
        }
    }

ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);

    /**
     * Registers the given driver with the {@code DriverManager}.
     * A newly-loaded driver class should call
     * the method {@code registerDriver} to make itself
     * known to the {@code DriverManager}. If the driver is currently
     * registered, no action is taken.
     *
     * @param driver the new JDBC Driver that is to be registered with the
     *               {@code DriverManager}
     * @param da     the {@code DriverAction} implementation to be used when
     *               {@code DriverManager#deregisterDriver} is called
     * @exception SQLException if a database access error occurs
     * @exception NullPointerException if {@code driver} is null
     * @since 1.8
     */
    public static synchronized void registerDriver(java.sql.Driver driver,
            DriverAction da)
        throws SQLException {

        /* Register the driver if it has not already been added to our list */
        if(driver != null) {
            registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
        } else {
            // This is for compatibility with the original DriverManager
            throw new NullPointerException();
        }

        println("registerDriver: " + driver);

    }

Connection conn = DriverManager.getConnection(url,username,password);

@CallerSensitive
    public static Connection getConnection(String url,
        String user, String password) throws SQLException {
        java.util.Properties info = new java.util.Properties();

        if (user != null) {
            info.put("user", user);
        }
        if (password != null) {
            info.put("password", password);
        }

        return (getConnection(url, info, Reflection.getCallerClass()));
    }

 //  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;
        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");
    }

    private static boolean isDriverAllowed(Driver driver, ClassLoader classLoader) {
        boolean result = false;
        if(driver != null) {
            Class aClass = null;
            try {
                aClass =  Class.forName(driver.getClass().getName(), true, classLoader);
            } catch (Exception ex) {
                result = false;
            }

             result = ( aClass == driver.getClass() ) ? true : false;
        }

        return result;
    }

Connection conn = DriverManager.getConnection(url,username,password);