Class.forName与ClassLoader

淩亂°似流年 2022-02-03 08:19 266阅读 0赞

先写一个简单的类

package org.test.three;
    
    public class Book {
    	
    	public Book(){
    		System.out.println("Here is Book construction.");
    	}
    	
    	public static String bookName="mybook";
    	private int price;
    	private String author;
    	private boolean isSale;
    	static{
    		System.out.println("Here is Book static area. The bookName is:"+bookName);
    	}
    	
    	public void printAllInfo(){
    		System.out.println("All info is[bookName:"+bookName+",author:"+author+",price:"+price+",isSale:"+isSale+"]");
    	}
    
    }

测试类

package org.test.three;
    
    /**
     * java中Class.forName和ClassLoader都可用来对类进行加载。
     * Class.forName除了将类的.class文件加载到jvm中之外，还会对类进行解释，执行类中的static块。
     * 而ClassLoader只将.class文件加载到jvm中，不会执行static中的内容,只有在newInstance才会去执行static块。
     * 
     * Class.forName(name, initialize, loader)中参函数initialize也可控制是否加载static块。
     * 调用newInstance()方法将调用构造函数，创建类的对象。
     * 
     * @author lushuaiyin
     *
     */
    public class TestMain {
    
    	/**
    	 * @param args
    	 */
    	public static void main(String[] args) {
    		//测试0
    //		Book bk0=new Book();
    		
            /Class.forName
    		
    //		try {
    //			//测试1
    			Class cc1=Class.forName("org.test.three.Book");
    //			
    //			//测试2
    			Class cc12=Class.forName("org.test.three.Book");
    			Book bk1=(Book)cc12.newInstance();
    			bk1.printAllInfo();
    //			
    //			//下面同上
    			@SuppressWarnings("unchecked")
    			Class<Book> cc13=(Class<Book>) Class.forName("org.test.three.Book");
    			Book bk2=cc13.newInstance();
    			bk2.printAllInfo();
    //			
    //			/*第二种Class.forName(name, initialize, loader)
    //			 name:加载的类名；initialize是否初始化，即是否执行static代码块和初始化变量值。loader：class loader
    //			 Class.forName("XXX") 等价于  Class.forName("XXX",true,XXX.class.getClassLoader());
    //			 */
    //			//测试3
    			Class cc14=Class.forName("org.test.three.Book",true,org.test.three.Book.class.getClassLoader());
    //			
    //			//测试4
    			Class cc15=Class.forName("org.test.three.Book",false,org.test.three.Book.class.getClassLoader());
    //			
    //			//测试5
    			Class cc16=Class.forName("org.test.three.Book",true,org.test.three.Book.class.getClassLoader());
    			Book bk3=(Book)cc16.newInstance();
    			bk3.printAllInfo();
    //			
    //			//测试6
    //			Class cc17=Class.forName("org.test.three.Book",false,org.test.three.Book.class.getClassLoader());
    //			Book bk4=(Book)cc17.newInstance();
    //			bk4.printAllInfo();
    //			
    //		} catch (ClassNotFoundException e) {
    //			e.printStackTrace();
    //		}
    //		catch (InstantiationException e) {
    //			e.printStackTrace();
    //		} catch (IllegalAccessException e) {
    //			e.printStackTrace();
    //		}
    		
    		/ClassLoader
    		try {
    			
    			//测试7
    //			Class cc21=ClassLoader.getSystemClassLoader().loadClass("org.test.three.Book");
    			
    			//测试8
    //			Class cc22=org.test.three.Book.class.getClassLoader().loadClass("org.test.three.Book");
    			
    			
    //			//测试9
    //			Class cc23=ClassLoader.getSystemClassLoader().loadClass("org.test.three.Book");
    //			Book bk5=(Book)cc23.newInstance();
    //			bk5.printAllInfo();
    			
    			//测试10
    //			Class cc24=Thread.currentThread().getContextClassLoader().loadClass("org.test.three.Book");
    //			Book bk6=(Book)cc24.newInstance();
    //			bk6.printAllInfo();
    			
    			//测试11
    			Class cc25=Thread.class.newInstance().getContextClassLoader().loadClass("org.test.three.Book");
    			Book bk7=(Book)cc25.newInstance();
    			bk7.printAllInfo();
    		} catch (ClassNotFoundException e) {
    			e.printStackTrace();
    		}
    		catch (InstantiationException e) {
    			e.printStackTrace();
    		} catch (IllegalAccessException e) {
    			e.printStackTrace();
    		}
    		
    	}
    
    }
    /*
    测试0打印
    Here is Book static area. The bookName is:mybook
    
    测试1打印
    Here is Book static area. The bookName is:mybook
    
    测试2打印
    Here is Book static area. The bookName is:mybook
    Here is Book construction.
    All info is[bookName:mybook,author:null,price:0,isSale:false]
    
    测试3打印
    Here is Book static area. The bookName is:mybook
    
    测试4打印
    (无输出内容)
    
    测试5打印
    Here is Book static area. The bookName is:mybook
    Here is Book construction.
    All info is[bookName:mybook,author:null,price:0,isSale:false]
    
    测试6打印
    Here is Book static area. The bookName is:mybook
    Here is Book construction.
    All info is[bookName:mybook,author:null,price:0,isSale:false]
    
    测试7打印
    (无输出内容)
    
    测试8打印
    (无输出内容)
    
    
    测试9打印
    Here is Book static area. The bookName is:mybook
    Here is Book construction.
    All info is[bookName:mybook,author:null,price:0,isSale:false]
    
    测试10打印
    Here is Book static area. The bookName is:mybook
    Here is Book construction.
    All info is[bookName:mybook,author:null,price:0,isSale:false]
    
    
    测试11打印
    Here is Book static area. The bookName is:mybook
    Here is Book construction.
    All info is[bookName:mybook,author:null,price:0,isSale:false]
    
    
    //链接数据库时，会调用下面代码，其实就是加载驱动类，执行其中的static块，注册驱动。
    //		Class.forName("com.mysql.jdbc.Driver");
    
    static 
        {
            try
            {
                DriverManager.registerDriver(new Driver());
            }
            catch(SQLException E)
            {
                throw new RuntimeException("Can't register driver!");
            }
        }
        
    //		Class.forName("oracle.jdbc.driver.OracleDriver");
    static 
        {
            defaultDriver = null;
            Timestamp timestamp = Timestamp.valueOf("2000-01-01 00:00:00.0");
            try
            {
                if(defaultDriver == null)
                {
                    defaultDriver = new OracleDriver();
                    DriverManager.registerDriver(defaultDriver);
                }
            }
            catch(RuntimeException runtimeexception) { }
            catch(SQLException sqlexception) { }
        }
     */