Java中线程上下文类加载器超详细讲解使用

package com.brycen.classloader;
import java.sql.Driver;
import java.util.Iterator;
import java.util.ServiceLoader;
public class MyTest26 {
    public static void main(String[] args) {
        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 class MyTest27 {
    public static void main(String[] args) throws ClassNotFoundException, SQLException {
    	//加载并初始化com.mysql.jdbc.Driver
        Class.forName("com.mysql.jdbc.Driver");
        Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "username", "password");
    }
}

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    public Driver() throws SQLException {
    }
	//静态代码块，初始化的时候会执行
    static {
        try {
        	//主动使用DriverManager，则该类也会初始化
        	//初始化完成后就调用DriverManager的registerDriver方法将自身添加到驱动集合中。
            DriverManager.registerDriver(new Driver());
        } catch (SQLException var1) {
            throw new RuntimeException("Can't register driver!");
        }
    }
}

public class DriverManager {
    // 注册JDBC驱动的集合
    private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
    ...
	...
    static {
    	//当初始化的时候会执行该方法
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }
    ...
    ...
    private static void loadInitialDrivers() {
        String drivers;
        //通过获取系统参数来加载jdbc的驱动，如果没有该参数则返回null
        try {
            drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
                public String run() {
                    return System.getProperty("jdbc.drivers");
                }
            });
        } catch (Exception ex) {
            drivers = null;
        }
        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {
				//通过ServiceLoader来加载驱动，ServiceLoader已经在上面讲解过了
                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                Iterator<Driver> driversIterator = loadedDrivers.iterator();
                try{
                	//这里会将加载到的驱动保存到上面的registeredDrivers集合中去
                    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);
            }
        }
    }
    ...
    ...

...
...
//这个方法在com.mysql.jdbc.Driver初始化的时候被调用
public static synchronized void registerDriver(java.sql.Driver driver)
    throws SQLException {
	//将驱动注册到registeredDrivers集合中去
    registerDriver(driver, null);
}
...
...

@CallerSensitive
public static Connection getConnection(String url,
    String user, String password) throws SQLException {
    java.util.Properties info = new java.util.Properties();
	//封装用户名和密码
    if (user != null) {
        info.put("user", user);
    }
    if (password != null) {
        info.put("password", password);
    }
	//调用getConnection，并把基本信息和调用者的class（这里就是我们的MyTest27.class）
	//Reflection.getCallerClass()是个本地方法，返回调用者的class
    return (getConnection(url, info, Reflection.getCallerClass()));
}
private static Connection getConnection(
    String url, java.util.Properties info, Class<?> caller) throws SQLException {
    //这里获取调用者的类加载器，如果为null则获取线程上下文类加载
    //从而实现能够在DirverManager中访问到放在我们classpath目录下的驱动
    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 + "\")");
    SQLException reason = null;
    for(DriverInfo aDriver : registeredDrivers) {
        //判断每一个驱动是否有权限，这里的权限就是判断该驱动的类加载器
        //和上面获取到的类加载器是否一致
        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");
}