java类的动态加载

电玩女神 2024-05-23 14:17 93阅读 0赞

## java类的动态加载 ##

**java动态加载的机制：**  
ClassLoader->SecureClassloader–>URLClassLoader–>AppClassLoader  
loadClass–>findClass(重写方法)–>defineClass(从字节码加载类)

初始化的时候会加载静态代码块  
实例化的时候会加载构造代码块、无参构造函数

#### 1、URLClassLoader动态加载任意类 ####

测试静态类

package loadClass;
    
    import java.io.IOException;
    
    public class test {
        
        static {
        
            try {
        
                Runtime.getRuntime().exec("calc");
            } catch (IOException e) {
        
                e.printStackTrace();
            }
        }
    }

测试动态加载：

package loadClass;
    
    import com.sun.jndi.toolkit.url.UrlUtil;
    import serializes.Person;
    
    import java.net.MalformedURLException;
    import java.net.URL;
    import java.net.URLClassLoader;
    
    public class loadclass_test {
        
        public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, MalformedURLException {
        
    
    		//file协议加载
            //URLClassLoader urlClassLoader = new URLClassLoader(new URL[] {new URL("file:///C:\\temp\\")});	//文件所在目录
            //http协议加载
            //URLClassLoader urlClassLoader = new URLClassLoader(new URL[] {new URL("http://127.0.0.1:888/")});
            //jar协议
            URLClassLoader urlClassLoader = new URLClassLoader(new URL[] {
        new URL("jar:file:///C:\\temp\\test.jar!/")});
            Class<?> loadClass = urlClassLoader.loadClass("loadClass.test");	//全名：包名+类名
            loadClass.newInstance();
    
        }
    }

执行完成截图：  
![在这里插入图片描述][270a4e87f3174c6090dc88c87e278523.png]

#### 2、defineClass 动态加载任意类 ####

ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();	//获取系统的类加载器（classloader）
            Method defineClass = ClassLoader.class.getDeclaredMethod("defineClass", String.class, byte[].class, int.class, int.class);
            defineClass.setAccessible(true);
            byte[] path = Files.readAllBytes(Paths.get("C:\\temp\\test.class"));
            Class test = (Class)defineClass.invoke(systemClassLoader, "loadClass.test", path, 0, path.length);
            test.newInstance();

#### 3、Unsave动态加载任意类: ####

ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
    		Class c = Unsafe.class;
            Field theUnsafe = c.getDeclaredField("theUnsafe");
            theUnsafe.setAccessible(true);
            Unsafe unsafe = (Unsafe)theUnsafe.get(null);
            byte[] path = Files.readAllBytes(Paths.get("C:\\temp\\test.class"));
            Class<?> test1 = unsafe.defineClass("loadClass.test", path, 0, path.length, systemClassLoader, null);
            test1.newInstance();

#### 编译方法： ####

javac test.java————>test.class  
![在这里插入图片描述][f8776f0b0e75445db09d2f1fea1ba64a.png]  
test.class——>test.jar  
打包成jar包：jar cvf 要生成的jar文件名 类名  
![在这里插入图片描述][99161ed24f04422ba81c745984d27837.png]

[270a4e87f3174c6090dc88c87e278523.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/05/23/a4d37731ccd94a5ea9fd80267fcb71db.png
[f8776f0b0e75445db09d2f1fea1ba64a.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/05/23/6278b55afd0140c7b3a9e3913e8dba87.png
[99161ed24f04422ba81c745984d27837.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/05/23/727f90d13f664235aea2358cff108552.png