为何说JDBC驱动类加载破坏了双亲委派机制

你们都知道jdk的类加载机制是双亲委派机制，当咱们须要加载一个类的时候，好比以下几种状况

• new一个类的对象
• 调用类的静态成员（除了final常量）和静态方法
• 使用java.lang.reflect包的方法对类进行反射调用
• 当虚拟机启动，java Demo01,则必定会初始化Demo01类，加载main方法所在的类
• 当初始化一个类，若是其父类没有被初始化，则先初始化它父类

那么看一段代码：

public static void main(String[] args) throws SQLException {
        String url = "jdbc:mysql://...";
        String username = "root";
        String password = "root";
        Connection conn = null;
        try {
            conn = DriverManager.getConnection(url, username, password);
            PreparedStatement statement = conn.prepareStatement("select * from table where id = ?");
            statement.setLong(1,615444000000000L);
            ResultSet set = statement.executeQuery();
            while(set.next()){
                System.out.println(set.getString("user_id"));
            }
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            if(conn!=null){
                try {
                    conn.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }
    }

上面是一段标准的jdbc查询代码，在加载java.sql.DriverManager类的时候，会执行静态块

static {
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }

主要是这两行代码

ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
Iterator<Driver> driversIterator = loadedDrivers.iterator();
//咱们看到这里使用了线程上下文加载器，为何这里要这么作？？若是不这么作会怎样？
    public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

下面是java.util.ServiceLoader$LazyIterator.nextService()方法

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
        }

咱们看到了Class.forName(cn, false, loader)的调用，若是前面没有用线程上下文加载器(也就是loader)，那么这里默认使用的是调用方的类加载器，以下所示：

/**
     * Returns the {@code Class} object associated with the class or
     * interface with the given string name.  Invoking this method is
     * equivalent to:
     *
     * <blockquote>
     *  {@code Class.forName(className, true, currentLoader)}
     * </blockquote>
     *
     * where {@code currentLoader} denotes the defining class loader of
     * the current class.
     *
     * <p> For example, the following code fragment returns the
     * runtime {@code Class} descriptor for the class named
     * {@code java.lang.Thread}:
     *
     * <blockquote>
     *   {@code Class t = Class.forName("java.lang.Thread")}
     * </blockquote>
     * <p>
     * A call to {@code forName("X")} causes the class named
     * {@code X} to be initialized.
     *
     * @param      className   the fully qualified name of the desired class.
     * @return     the {@code Class} object for the class with the
     *             specified name.
     * @exception LinkageError if the linkage fails
     * @exception ExceptionInInitializerError if the initialization provoked
     *            by this method fails
     * @exception ClassNotFoundException if the class cannot be located
     */
    @CallerSensitive
    public static Class<?> forName(String className)
                throws ClassNotFoundException {
        Class<?> caller = Reflection.getCallerClass();
        return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
    }

那么若是没有使用线程上下文加载器，其实用的就是调用方 ServiceLoader 的加载器，而ServiceLoader在java.util下面，是没法加载com.mysql.jdbc包下的类。 我不认为这是对双亲委派的破坏。