【ClassLoader】实现自定义类加载器加载指定路径下的Class文件和Jar包[通俗易懂]

/** * @author cry777 * @description * @create 2022-03-02 */
public class MyClassLoader extends ClassLoader { 

private String classPath;
public AgentClassLoader(String classPath) { 

this.classPath = classPath;
}
private byte[] loadByte(String name) throws Exception { 

name = name.replaceAll("\\.", "/");
FileInputStream fis = new FileInputStream(classPath + "/" + name
+ ".class");
int len = fis.available();
byte[] data = new byte[len];
fis.read(data);
fis.close();
return data;
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException { 

try { 

byte[] data = loadByte(name);
return defineClass(name, data, 0, data.length);
} catch (Exception e) { 

e.printStackTrace();
throw new ClassNotFoundException();
}
}
}

public class User { 

private int id;
private String name;
public User() { 

}
public User(int id, String name) { 

super();
this.id = id;
this.name = name;
}
public int getId() { 

return id;
}
public void setId(int id) { 

this.id = id;
}
public String getName() { 

return name;
}
public void setName(String name) { 

this.name = name;
}
public void sout() { 

System.out.println("=======自己的加载器加载类调用方法=======");
}
public static void main(String[] args) { 

User user = new User();
}
}

public class MyClassLoaderTest { 

public static void main(String args[]) throws Exception { 

MyClassLoader classLoader = new MyClassLoader ("D:\\test");
Class clazz = classLoader.loadClass("com.cry.User");
Object obj = clazz.newInstance();
Method method = clazz.getDeclaredMethod("sout", null);
method.invoke(obj, null);
System.out.println(clazz.getClassLoader().getClass().getName());
}
}

public String classNameToJarEntry(String name){ 

String s = name.replaceAll("\\.", "\\/");
StringBuilder stringBuilder = new StringBuilder(s);
stringBuilder.append(".class");
return stringBuilder.toString();
}

public class JarClassLoader extends ClassLoader { 

public JarFile jarFile;
public ClassLoader parent;
public JarClassLoader(JarFile jarFile) { 

super(Thread.currentThread().getContextClassLoader());
this.parent = Thread.currentThread().getContextClassLoader();
this.jarFile = jarFile;
}
public JarClassLoader(JarFile jarFile, ClassLoader parent) { 

super(parent);
this.parent = parent;
this.jarFile = jarFile;
}
public String classNameToJarEntry(String name){ 

String s = name.replaceAll("\\.", "\\/");
StringBuilder stringBuilder = new StringBuilder(s);
stringBuilder.append(".class");
return stringBuilder.toString();
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException { 

try { 

Class c = null;
if (null != jarFile) { 

String jarEntryName = classNameToJarEntry(name);
JarEntry entry = jarFile.getJarEntry(jarEntryName);
if (null != entry) { 

InputStream is = jarFile.getInputStream(entry);
int availableLen = is.available();
int len = 0;
byte[] bt1 = new byte[availableLen];
while (len < availableLen) { 

len += is.read(bt1, len, availableLen - len);
}
c = defineClass(name, bt1, 0, bt1.length);
} else { 

if (parent != null) { 

return parent.loadClass(name);
}
}
}
return c;
} catch (IOException e) { 

throw new ClassNotFoundException("Class " + name + " not found.");
}
}
@Override
public InputStream getResourceAsStream(String name) { 

InputStream is = null;
try { 

if (null != jarFile) { 

JarEntry entry = jarFile.getJarEntry(name);
if (entry != null) { 

is = jarFile.getInputStream(entry);
}
if (is == null) { 

is = super.getResourceAsStream(name);
}
}
} catch (IOException e) { 

// logger.error(e.getMessage());
System.out.println(e.getMessage());
}
return is;
}
}

public static void main(String[] args) throws IOException, ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException { 

JarClassLoader jarClassLoader = new JarClassLoader(new JarFile(new File("D:\\test\\test.jar")));
Enumeration<JarEntry> entries = jarClassLoader.jarFile.entries();
while (entries.hasMoreElements()) { 

String name = entries.nextElement().getName();
System.out.println("name: " + name);
}
Class clazz1 = jarClassLoader.loadClass("User");
Object obj1 = clazz1.newInstance();
Method method1 = clazz1.getDeclaredMethod("sout", null);
method1.invoke(obj1, null);
System.out.println(clazz1.getClassLoader().getClass().getName());
}

public static MyInterface createAgentInstance(ClassLoader myClassLoader,String entryClass) throws Exception{ 

MyInterface myInstance = null;
Thread currentThread = Thread.currentThread();
ClassLoader beforeClassLoader = currentThread.getContextClassLoader();
currentThread.setContextClassLoader(entryClass);
try { 

Class<?> entryClass= myClassLoader.loadClass(entryClass);
Constructor<?> constructor = entryClass.getDeclaredConstructor();
Object instance = constructor.newInstance();
if (instance instanceof MyInterface ){ 

myInstance = (MyInterface ) instance;
}
} finally { 

currentThread.setContextClassLoader(beforeClassLoader);
}
return myInstance ;
}