反射
知识基础
- 类
- 对象
- 构造方法
- 重载
- 单例模式
动态加载
JVM在执行的时候,并不是一次性把所有的class加载到内存中的,而是用到谁加载谁。
反射的概述
https://www.cnblogs.com/tech-bird/p/3525336.html
Reflection(反射)是Java被视为动态语言的关键,反射机制允许程序在执行期借助于Reflection API取得任何类的內部信息,并能直接操作任意对象的内部属性及方法
获取class对象的三种方式
- class.forName(“全类名”)
- 类名.class
- 对象.getClass(),如果上下文存在某个实例对象,可以通过getClass获取他的类
public class re {
public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
Class cls1 = Class.forName(\"demo.Person\");
Class cls2 = Person.class;
Person person = new Person();
Class cls3 = person.getClass();
System.out.println(cls1);
System.out.println(cls2);
System.out.println(cls3);
System.out.println(cls1==cls2);
System.out.println(cls3==cls2);
}
类“初始化”执行顺序是什么
package demo;
public class test {
public static void main(String[] args) {
Ref ref = new Ref();
}
}
class Ref{
static {
System.out.println(\"最先执行\\r\\n\");
}
{
System.out.println(\"第二执行\\r\\n\");
}
public Ref(){
System.out.println(\"最后执行\\r\\n\");
}
}
demo1
package demo;
public class test {
public static void main(String[] args) throws ClassNotFoundException {
Class.forName(\"demo.CalcDemo\");
}
}
class CalcDemo {
static {
try {
Runtime rt = Runtime.getRuntime();
Process pc = rt.exec(\"calc\");
pc.waitFor();
} catch (Exception e) {
}
}
}
Class方法
获取变量
- Field getField(name):根据字段名获取某个public的field(包括父类)
- Field getDeclaredField(name):根据字段名获取当前类的某个field(不包括父类)
- Field[] getFields():获取所有public的field(包括父类)
- Field[] getDeclaredFields():获取当前类的所有field(不包括父类)
cls3.getField(\"aaa\");//指定名称的public修饰的
cls3.getFields();//获取所有public修饰的成员变量
cls3.getDeclaredField(\"aaaa\");//获取所有
cls3.getDeclaredFields();
获取构造方法
cls3.getConstructors();
cls3.getConstructor(\"aaa\");
cls3.getDeclaredConstructor(\"bbb\");
cls3.getDeclaredConstructors();
package demo;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
public class test {
public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
// Ref ref = new Ref();
Class cls = Class.forName(\"demo.Person\");
Constructor constructor = cls.getConstructor();
Constructor constructor1 = cls.getConstructor(String.class);
constructor.newInstance();
constructor1.newInstance(\"jl\");
}
}
demo2
package demo;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.List;
public class test {
public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
Class cls = Class.forName(\"java.lang.ProcessBuilder\");
Method methodStart = cls.getMethod(\"start\");
Constructor constructor = cls.getConstructor(List.class);
Object obj = constructor.newInstance(Arrays.asList(\"calc.exe\"));
methodStart.invoke(obj);
}
}
demo3-可变长参数
package demo;import java.lang.reflect.Constructor;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;import java.util.Arrays;import java.util.List;public class test { public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException { Class cls = Class.forName(\"java.lang.ProcessBuilder\"); Method methodStart = cls.getMethod(\"start\"); Constructor constructor = cls.getConstructor(String[].class); Object obj = constructor.newInstance(new String[][]{{\"calc.exe\"}}); methodStart.invoke(obj); }}
获取成员方法
cls3.getMethods();cls3.getMethod(\"ccc\");cls3.getDeclaredMethods();cls3.getDeclaredMethod(\"ccc\");
demo4
package demo;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;public class test { public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException { Person person = new Person(); Class cls = Class.forName(\"demo.Person\"); Method method = cls.getDeclaredMethod(\"eat\", String.class); method.setAccessible(true); method.invoke(person,\"emmmm\"); }}
demo5
package demo;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;public class test { public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException { Class cls = Class.forName(\"java.lang.Runtime\"); Method method = cls.getMethod(\"exec\", String.class); Method method1 = cls.getMethod(\"getRuntime\"); method.invoke(method1.invoke(cls),\"calc.exe\"); }}
Runtime类就是单例模式,我们只能通过 Runtime.getRuntime() 来获取到 Runtime 对
象。
最新评论
Thanks for your blog, nice to read. Do not stop.
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