Java中執行緒上下文類載入器超詳細講解使用

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()+", 類載入器："+dirver.getClass().getClassLoader());
        }
        System.out.println("當前執行緒上線文類載入器："+Thread.currentThread().getContextClassLoader());
        System.out.println("ServiceLoader類載入器："+loader.getClass().getClassLoader());
    }
}

public class MyTest27 {
    public static void main(String[] args) throws ClassNotFoundException, SQLException {
    	//載入並初始化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 {
    }
	//靜態程式碼塊，初始化的時候會執行
    static {
        try {
        	//主動使用DriverManager，則該類也會初始化
        	//初始化完成後就呼叫DriverManager的registerDriver方法將自身新增到驅動集合中。
            DriverManager.registerDriver(new Driver());
        } catch (SQLException var1) {
            throw new RuntimeException("Can't register driver!");
        }
    }
}

public class DriverManager {
    // 註冊JDBC驅動的集合
    private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
    ...
	...
    static {
    	//當初始化的時候會執行該方法
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }
    ...
    ...
    private static void loadInitialDrivers() {
        String drivers;
        //通過獲取系統引數來載入jdbc的驅動，如果沒有該引數則返回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() {
				//通過ServiceLoader來載入驅動，ServiceLoader已經在上面講解過了
                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                Iterator<Driver> driversIterator = loadedDrivers.iterator();
                try{
                	//這裡會將載入到的驅動儲存到上面的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);
            }
        }
    }
    ...
    ...

...
...
//這個方法在com.mysql.jdbc.Driver初始化的時候被呼叫
public static synchronized void registerDriver(java.sql.Driver driver)
    throws SQLException {
	//將驅動註冊到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();
	//封裝使用者名稱和密碼
    if (user != null) {
        info.put("user", user);
    }
    if (password != null) {
        info.put("password", password);
    }
	//呼叫getConnection，並把基本資訊和呼叫者的class（這裡就是我們的MyTest27.class）
	//Reflection.getCallerClass()是個本地方法，返回撥用者的class
    return (getConnection(url, info, Reflection.getCallerClass()));
}
private static Connection getConnection(
    String url, java.util.Properties info, Class<?> caller) throws SQLException {
    //這裡獲取呼叫者的類載入器，如果為null則獲取執行緒上下文類載入
    //從而實現能夠在DirverManager中存取到放在我們classpath目錄下的驅動
    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) {
        //判斷每一個驅動是否有許可權，這裡的許可權就是判斷該驅動的類載入器
        //和上面獲取到的類載入器是否一致
        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");
}