从SPI在JDBC中的应用

定义

什么是SPI

• SPI全称是Service Provider Interface(服务提供接口)，是jdk提供的服务发现机制，例如JDBC就始终它摆脱了旧版本需要手动注册的动作。

static {
   Class.forName("com.mysql.jdbc.Driver")
}

JDBC实现

说明

• 旧版本需要手动注册Driver类

static {
   Class.forName("com.mysql.jdbc.Driver")
}

• 新版本则不需要这个动作

public class Demo {
    //旧版本
    static {
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
    public static void main(String[] args) throws SQLException {
        final String jdbcUrl = "jdbc:mysql://localhost:3306/mysql";
        final String userName = "root";
        final String password = "root";

        try (Connection connection = DriverManager.getConnection(jdbcUrl, userName, password);
             Statement statement = connection.createStatement();
             ResultSet resultSet = statement.executeQuery("select * from user ")) {
            while (resultSet.next()){
                System.out.println(resultSet.getObject(0));
            }
        }
    }
}

逐步剖析源代码

• 先理清代码执行顺序 Demo=》DriverManager=》Driver =》DriverManager.registerDriver(new Driver)
• loadInitialDrivers 触发静态代码块

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

• DriverManage.java 583行

 AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {
        //会去找META-INF.services.java.sql.Driver
                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                Iterator<Driver> driversIterator = loadedDrivers.iterator();
        try{
                    while(driversIterator.hasNext()) {
            //这里会将com.mysql.cj.jdbc.Driver进行实例化，从而触发Driver的静态代码块执行
                        driversIterator.next();
                    }
                } catch(Throwable t) {
                // Do nothing
                }
                return null;
            }
        });

• com.mysql.cj.jdbc.Driver

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!");
        }
    }
}

• DriverManager.java

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) {
        //如果driver不存在，则注册一个driver到列表里面
            registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
        } else {
            // This is for compatibility with the original DriverManager
            throw new NullPointerException();
        }

        println("registerDriver: " + driver);

    }

• DriverManage.getConnection时会从registeredDrivers列表中取driver进行链接

优点

• 解偶，通过SPI，可以使第三方服务和本地服务分离

缺点

• 要遍历所有的实现类，并且都实例化

双亲委派模型

什么是双亲委派模型

protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
            //自己不去加载，调用父亲的loadClass方法，让父亲去加载
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);

                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

SPI破坏了双亲委派模型？

• 是的，因为DriverManager是在rt包下，应该是由顶层的加载器(bootstrap ClaasLoader进行加载),但是双亲委派模型有个缺点(只能请求父亲加载，不能父亲请求儿子加载)，这样bootstrap加载器会找不到com.mysql.cj.jdbc.Driver
• 为了解决这中情况，java设计人员提供了一个办法，在Thread类中，添加contextClassLoader(最底层的类加载器)，由这个加载器走双亲委派模型,这样就能完成加载了。

public static <S> ServiceLoader<S> load(Class<S> service) {
    //调用当前线程的类加载器
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }