问题：实例化对象时一定要用new调用构造方法吗？
答案：用普通的创建方法（new）是要调用构造方法，如果使用反射或者克隆这些就可以绕过构造函数直接创建对象
实例：
/* * ClassTest.java -- JDK 1.8 */ package T20190328; import java.io.File; import java.io.IOException; import java.lang.reflect.AccessibleObject; import java.lang.reflect.Constructor; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Type; import java.util.ArrayList; import java.util.List; import javax.xml.parsers.DocumentBuilder; import javax.xml.parsers.DocumentBuilderFactory; import javax.xml.parsers.ParserConfigurationException; import org.junit.Test; import org.w3c.dom.Document; import org.w3c.dom.Element; import org.w3c.dom.Node; import org.w3c.dom.NodeList; import org.xml.sax.SAXException; /** * Description: * <p> * 通过反射访问私有构造方法 例如: * * <pre> * <code> * // 1.获取类A中声明的构造器 * Class clazz = Class.forName("T20190328.A"); * Constructor constructor = clazz.getDeclaredConstructor(); // 假设A中只有一个私有构造方法 * // 2.设置私有访问权限 * constructor.setAccessible(true); * constructor.newInstance(); * </code> * </pre> * <p> * @author shuvidora * @date 2019-03-28 Thu PM 14:22:07 */ public class ClassTest { static List<A> list = new ArrayList<A>(); @SuppressWarnings("unchecked") public static <T> T[] cast(Object[] obj) { return (T[]) obj; } public static void showTypes(Type[] arr) { int len = arr.length; if (len == 0) { System.out.print("无元素"); return; } for (int i = 0; i < len; i++) { if (i == 0) System.out.print("("); if (i == len - 1) System.out.print(arr[i] + ")"); else System.out.print(arr[i] + ","); } } @SuppressWarnings("unchecked") @Test public void test() throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException { Constructor<A> constructor = null; try { constructor = (Constructor<A>) Class.forName("T20190328.A") .getDeclaredConstructor(String.class, String.class); } catch (NoSuchMethodException e) { e.printStackTrace(); } catch (SecurityException e) { e.printStackTrace(); } catch (ClassNotFoundException e) { e.printStackTrace(); } constructor.setAccessible(true); constructor.newInstance("x", "y"); } String clsPathFromXml() throws ParserConfigurationException, SAXException, IOException { DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); // 创建DOM解析器工厂对象 DocumentBuilder builder = factory.newDocumentBuilder(); // 创建DOM解析器对象 String path = this.getClass().getResource("om.xml").getPath(); Document doc = builder.parse(new File(path)); // 解析XML文档，并获取该XML文档的Document对象 Element component = doc.getDocumentElement(); // 获取根节点元素的Element对象 NodeList nodeList = component.getElementsByTagName("package"); Node pkg = nodeList.item(0); Node pkgAttr = pkg.getAttributes() .getNamedItem("name"); String pkgName = pkgAttr.getNodeValue(); Node cls = pkg.getFirstChild(); String clsName = cls.getNodeValue(); return pkgName + "." + clsName; } @Test public void testXml() { try { System.out.println(clsPathFromXml()); } catch (ParserConfigurationException | SAXException | IOException e) { e.printStackTrace(); } } public static void main(String[] args) { String className = null; ClassTest ct = new ClassTest(); try { className =ct.clsPathFromXml(); } catch (ParserConfigurationException | SAXException | IOException e1) { e1.printStackTrace(); } Constructor<A>[] constructors = null; try { constructors = cast(Class.forName(className) // 从类路径下加载并连接类T20190328.A .getDeclaredConstructors()); } catch (SecurityException | ClassNotFoundException e) { e.printStackTrace(); } AccessibleObject.setAccessible(constructors, true); // 批量授予访问私有访问权限 // 按类中定义的顺序输出构造器 for (int i = 0; i < constructors.length; i++) { Constructor<A> constructor = constructors[i]; System.out.print(constructor + "--->"); // 构造器参数全部类型 Type[] types = constructor.getGenericParameterTypes(); int typeNum = types.length; showTypes(types); System.out.print("===>"); // constructor.setAccessible(true); // 允许私有访问权限, 否则无法实例化对象 // 实例化对象 A a; try { if (typeNum == 1) { a = (A) constructor.newInstance(1); a.b(); } else if (typeNum == 2) { a = (A) constructor.newInstance("hello", "world"); a.b(); } else { a = (A) constructor.newInstance(); a.b(); } list.add(a); } catch (InstantiationException | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) { e.printStackTrace(); } } // 打印生成对象的内存地址哈希值 for (A a : list) { System.out.println(a); } } } class A extends B { private A(String x, String y) { System.out.println("has two params : " + x + "," + y); } private A(int i) { System.out.println("has one param : " + i); } private A() { System.out.println("has zero param"); } // private A(String... strings) { // System.out.println(strings); // } public void a() { System.out.println("A"); } } class B { String name; int age; public void b() { System.out.println("B"); } } om.xml
<?xml version="1.0" encoding="UTF-8"?> <component> <package name="T20190328"> <class>A </class> </package> </component>
问题来源：
https://www.nowcoder.com/questionTerminal/a3f14638ac2e432b884351840f42d80f?toCommentId=1111154