Class.forName（）详细介绍

Class.forName( )静态方法的目的是为了动态加载类。在加载完成后，一般还要调用Class下的newInstance( )静态方法来实例化对象以便操作。因此，Class.forName( )是动态加载类是没有用的，其最终目的是为了实例化对象。调用 forName("X") 将导致命名为 X 的类被初始化。

package com.java.jvm;

public class ClassTest {
	 public  static void main(String[] arg){ 
		 try {
			 Class  userClass = Class.forName("com.java.jvm.User");
			 System.out.println("userClass="+userClass);
			 User user = (User) userClass.newInstance();
			 System.out.println("user="+user);
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		} catch (InstantiationException e) {
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			e.printStackTrace();
		}
	  } 
}

userClass=class com.java.jvm.User
user=User [name=null, age=null, address=null]

调用 forName("X") 将导致命名为 X 的类被初始化，这句话是官网API上的一句话，很迷茫，有点不理解，所以在User类中，添加了static语句块和User构造函数

package com.java.jvm;

public class User {
	
	static{
		System.out.println("static ---");
	}
	
	public User(){
		System.out.println("name="+name);
		name="xiaotian";
	}
	
	private String name;
	private Integer age;
	private String address;
	
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public Integer getAge() {
		return age;
	}
	public void setAge(Integer age) {
		this.age = age;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address = address;
	}
	@Override
	public String toString() {
		return "User [name=" + name + ", age=" + age + ", address=" + address
				+ "]";
	}
	
}

static ---
userClass=class com.java.jvm.User
name=null
user=User [name=xiaotian, age=null, address=null]

forName("X") 执行的时候，调用了User中的静态语句块，newInstance()执行的时候调用了构造函数。

连接数据库的简单代码

// 驱动程序名
           String driver = "com.mysql.jdbc.Driver";

           // URL指向要访问的数据库名student
           String url = "jdbc:mysql://127.0.0.1:3306/student";

           // MySQL配置时的用户名
           String user = "root"; 

           // MySQL配置时的密码
           String password = "root";

           try { 
            // 加载驱动程序
            Class.forName(driver);

            // 连续数据库
            Connection conn = DriverManager.getConnection(url, user, password);

            if(!conn.isClosed()) 
             System.out.println("Succeeded connecting to the Database!");

            // statement用来执行SQL语句
            Statement statement = conn.createStatement();

            // 要执行的SQL语句
            String sql = "select * from student";

            // 结果集
            ResultSet rs = statement.executeQuery(sql);

Class.forName(driver);加载mysql驱动，执行static语句块，使将Driver对象注册到DriverManager中，然后上面就获得了数据库连接

package com.mysql.jdbc   
 
public class Driver extends NonRegisteringDriver implements java.sql.Driver {   
// ~ Static fields/initializers   
// --------------------------------------------- //   A
// Register ourselves with the DriverManager   
//   
static {   
     t ry {   //使将Driver对象注册到DriverManager中
               java.sql.DriverManager.registerDriver(new Driver());   
           } catch (SQLException E) {   
              throw new RuntimeException("Can't register driver!");   
           }   
   }   
// ~ Constructors   
// -----------------------------------------------------------   
/**
   * Construct a new driver and register it with DriverManager
   *
   * @throws SQLException
   *              if a database error occurs.
   */  
public Driver() throws SQLException {   
     // Required for Class.forName().newInstance()   
}   
}