1. Class loader

Class 1.1 loader [understanding]

• effect

  Be responsible for loading. class files (stored physical files) into memory

1.2 class loading process [understanding]

• Class loading timing

  • Create an instance (object) of a class
  • Call the class method of the class
  • Access or assign a value to a class variable of a class or interface
  • Use reflection to force the creation of java.lang.Class objects corresponding to a class or interface
  • Initializes a subclass of a class
  • Directly use the java.exe command to run a main class

• Class loading process

  1. load

     • Obtain this class through package name + class name, and prepare to use stream for transmission
     • After this class is loaded into memory
     • After loading, create a class object

1. link

   • verification

     Ensure that the information contained in the byte stream of the Class file meets the requirements of the current virtual machine and will not endanger the security of the virtual machine

     (whether the information in the file complies with the virtual machine specification, and whether there are security risks)

• prepare

  It is responsible for allocating memory for class variables (variables modified by static) of the class and setting the default initialization value

  (initialize static variables)

• analysis

  Replace the symbolic reference in the binary data stream of the class with a direct reference

  (if other classes are used in this class, you need to find the corresponding class.)

1. initialization

   Initialize class variables and other resources according to the subjective plan made by the programmer through the program

   (static variable assignment and initialization of other resources)

• Summary

  • When a class is used, it will be loaded into memory
  • Class loading process: loading, validation, preparation, parsing and initialization

1.3 classification of class loading [understanding]

• classification

  • Bootstrap class loader: the built-in class loader of the virtual machine, which is usually expressed as null and has no parent null
  • Platform class loader: platform class loader, which is responsible for loading some special modules in JDK
  • System class loader: system class loader, which is responsible for loading the class library specified on the user's class path

• Inheritance relationship of class loader

  • The parent loader of System is Platform
  • The parent loader of the Platform is Bootstrap

• Code demonstration

  public class ClassLoaderDemo1 {
      public static void main(String[] args) {
          //Get system class loader
          ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
  
          //Get the parent loader of the system class loader - platform class loader
          ClassLoader classLoader1 = systemClassLoader.getParent();
  
          //Get the parent loader of the platform class loader --- start the class loader
          ClassLoader classLoader2 = classLoader1.getParent();
  
          System.out.println("system class loader " + systemClassLoader);
          System.out.println("Platform class loader" + classLoader1);
          System.out.println("Start class loader" + classLoader2);
  
      }
  }
  

1.4 parental delegation model [understanding]

• introduce

  If a class loader receives a class loading request, it will not load it first, but delegate the request to the loader of the parent class for execution. If the parent class loader still has its parent class loader, it will delegate upward and recurse in turn. The request will eventually reach the top-level startup class loader. If the parent class loader can complete the class loading task, If the parent loader fails to complete the loading task, the child loader will try to load it by itself. This is the parent delegation mode

1.5 two methods in classloader [application]

• Method introduction

  Method name	explain
  public static ClassLoader getSystemClassLoader()	Get system class loader
  public InputStream getResourceAsStream(String name)	Load a resource file

• Sample code

  public class ClassLoaderDemo2 {
      public static void main(String[] args) throws IOException {
          //static ClassLoader getSystemClassLoader() gets the system class loader
          //InputStream getResourceAsStream(String name) loads a resource file
  
          //Get system class loader
          ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
  
          //Use the loader to load a specified file
          //Parameter: the path of the file (placed in the root directory of src and loaded there by default)
          //Return value: byte stream.
          InputStream is = systemClassLoader.getResourceAsStream("prop.properties");
  
          Properties prop = new Properties();
          prop.load(is);
  
          System.out.println(prop);
  
          is.close();
      }
  }
  

2. Reflection

2.1 overview of reflection [understanding]

• Reflection mechanism

  In the running state, you can know all the properties and methods of any class;
  For any object, you can call any of its properties and methods;

  This function of dynamically obtaining information and dynamically calling object methods is called the reflection mechanism of Java language.

2.2 three methods of obtaining Class objects [ application ]

• Three ways of classification

  • class name. class attribute

  • Object name. getClass() method

  • Class. Forname (full class name) method

• Sample code

  public class Student {
      private String name;
      private int age;
  
      public Student() {
      }
  
      public Student(String name, int age) {
          this.name = name;
          this.age = age;
      }
  
      public String getName() {
          return name;
      }
  
      public void setName(String name) {
          this.name = name;
      }
  
      public int getAge() {
          return age;
      }
  
      public void setAge(int age) {
          this.age = age;
      }
  
      public void study(){
          System.out.println("Students are studying");
      }
  
      @Override
      public String toString() {
          return "Student{" +
                  "name='" + name + '\'' +
                  ", age=" + age +
                  '}';
      }
  }
  public class ReflectDemo1 {
      public static void main(String[] args) throws ClassNotFoundException {
          //1. Static method forName("full class name") in class
              //Full class name: package name + class name
          Class clazz = Class.forName("com.XXX.myreflect2.Student");
          System.out.println(clazz);
  
          //2. Get through class attribute
          Class clazz2 = Student.class;
          System.out.println(clazz2);
  
          //3. Use the getClass method of the object to get the class object
          //The getClass method is defined in the Object class
          Student s = new Student();
          Class clazz3 = s.getClass();
          System.out.println(clazz3);
  
          System.out.println(clazz == clazz2);
          System.out.println(clazz2 == clazz3);
      }
  }
  

2.3 reflection acquisition construction method and use [application]

2.3.1Class class gets the method of constructing method objects

• Method introduction

  Method name	explain
  Constructor<?>[] getConstructors()	Returns an array of all public constructor objects
  Constructor<?>[] getDeclaredConstructors()	Returns an array of all constructor objects
  Constructor getConstructor(Class<?>... parameterTypes)	Returns a single common constructor object
  Constructor getDeclaredConstructor(Class<?>... parameterTypes)	Returns a single constructor object

• Sample code

  public class Student {
      private String name;
      private int age;
  
      //Private parametric construction method
      private Student(String name) {
          System.out.println("name The value of is:" + name);
          System.out.println("private...Student...Parametric construction method");
      }
  
      //Public parameterless construction method
      public Student() {
          System.out.println("public...Student...Nonparametric construction method");
      }
  
      //Public parametric construction method
      public Student(String name, int age) {
          System.out.println("name The value of is:" + name + "age The value of is:" + age);
          System.out.println("public...Student...Parametric construction method");
      }
  }
  public class ReflectDemo1 {
      public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException {
          //method1();
          //method2();
          //method3();
          //method4();
      }
  
      private static void method4() throws ClassNotFoundException, NoSuchMethodException {
          //        Constructor<T> getDeclaredConstructor(Class<?>... parameterTypes): 
  //                                      Returns a single constructor object
          //1. Get Class object
          Class clazz = Class.forName("com.XXX.myreflect3.Student");
          Constructor constructor = clazz.getDeclaredConstructor(String.class);
          System.out.println(constructor);
      }
  
      private static void method3() throws ClassNotFoundException, NoSuchMethodException {
          //        Constructor<T> getConstructor(Class<?>... parameterTypes): 
  //                                      Returns a single common constructor object
          //1. Get Class object
          Class clazz = Class.forName("com.XXX.myreflect3.Student");
          //In parentheses, it must be consistent with the formal parameters of the constructor
          Constructor constructor1 = clazz.getConstructor();
          System.out.println(constructor1);
  
          Constructor constructor2 = clazz.getConstructor(String.class, int.class);
          System.out.println(constructor2);
  
          //Because there is no construct with only one int in the Student class, an error will be reported here
          Constructor constructor3 = clazz.getConstructor(int.class);
          System.out.println(constructor3);
      }
  
      private static void method2() throws ClassNotFoundException {
          //        Constructor<?>[] getDeclaredConstructors(): 
  //                                      Returns an array of all constructor objects
          //1. Get Class object
          Class clazz = Class.forName("com.XXX.myreflect3.Student");
  
          Constructor[] constructors = clazz.getDeclaredConstructors();
          for (Constructor constructor : constructors) {
              System.out.println(constructor);
          }
      }
  
      private static void method1() throws ClassNotFoundException {
          //        Constructor<?>[] getConstructors(): 
  //                                      Returns an array of all public constructor objects
          //1. Get Class object
          Class clazz = Class.forName("com.XXX.myreflect3.Student");
          Constructor[] constructors = clazz.getConstructors();
          for (Constructor constructor : constructors) {
              System.out.println(constructor);
          }
      }
  }
  

2.3.2Constructor class method for creating objects

• Method introduction

  Method name	explain
  T newInstance(Object...initargs)	Creates an object according to the specified construction method
  setAccessible(boolean flag)	Set to true to cancel the access check

• Sample code

  // The Student class is the same as the previous example, which is not repeated here
  public class ReflectDemo2 {
      public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
          //T newInstance(Object... initargs): creates an object according to the specified construction method
          //method1();
          //method2();
          //method3();
          //method4();
  
      }
  
      private static void method4() throws ClassNotFoundException, NoSuchMethodException, InstantiationException, IllegalAccessException, InvocationTargetException {
          //Get a private constructor and create an object
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect3.Student");
  
          //2. Obtain a privatized construction method
          Constructor constructor = clazz.getDeclaredConstructor(String.class);
  
          //Members decorated with private cannot be used directly
          //If you use reflection to forcibly obtain and use, you need to temporarily cancel the access check
          constructor.setAccessible(true);
  
          //3. Create objects directly
          Student student = (Student) constructor.newInstance("zhangsan");
  
          System.out.println(student);
      }
  
      private static void method3() throws ClassNotFoundException, InstantiationException, IllegalAccessException {
          //Abbreviation format
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect3.Student");
  
          //2. In Class, there is a newInstance method, which can directly create an object with null parameters
          Student student = (Student) clazz.newInstance();//This method is out of date now. Learn about it
  
          System.out.println(student);
      }
  
      private static void method2() throws ClassNotFoundException, NoSuchMethodException, InstantiationException, IllegalAccessException, InvocationTargetException {
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect3.Student");
  
          //2. Get construction method object
          Constructor constructor = clazz.getConstructor();
  
          //3. Create Student object with null parameter
          Student student = (Student) constructor.newInstance();
  
          System.out.println(student);
      }
  
      private static void method1() throws ClassNotFoundException, NoSuchMethodException, InstantiationException, IllegalAccessException, InvocationTargetException {
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect3.Student");
  
          //2. Get construction method object
          Constructor constructor = clazz.getConstructor(String.class, int.class);
  
          //3. Use newInstance to create the object of Student
          Student student = (Student) constructor.newInstance("zhangsan", 23);
  
          System.out.println(student);
      }
  }
  

2.3.3 summary

• Get class object

  Three methods: Class.forName("full class name"), class name. Class, object name. getClass()

• Get the constructor object inside

  getConstructor (Class<?>... parameterTypes)
  getDeclaredConstructor (Class<?>... parameterTypes)

• If it is public, create the object directly

  newInstance(Object... initargs)

• If it is not public, you need to cancel the check temporarily before creating the object

  setAccessible(boolean) violent reflection

2.4 get member variables by reflection and use [application]

2.4.1 class class method for obtaining member variable object

• Method classification

  Method name	explain
  Field[] getFields()	Returns an array of all public member variable objects
  Field[] getDeclaredFields()	Returns an array of all member variable objects
  Field getField(String name)	Returns a single public member variable object
  Field getDeclaredField(String name)	Returns a single member variable object

• Sample code

  public class Student {
  
      public String name;
  
      public int age;
  
      public String gender;
  
      private int money = 300;
  
      @Override
      public String toString() {
          return "Student{" +
                  "name='" + name + '\'' +
                  ", age=" + age +
                  ", gender='" + gender + '\'' +
                  ", money=" + money +
                  '}';
      }
  }
  public class ReflectDemo1 {
      public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
         // method1();
          //method2();
          //method3();
          //method4();
  
      }
  
      private static void method4() throws ClassNotFoundException, NoSuchFieldException {
          //        Field getdeclaraedfield (string name): returns a single member variable object
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect4.Student");
    
          //2. Get the money member variable
          Field field = clazz.getDeclaredField("money");
    
          //3. Print it
          System.out.println(field);
      }
    
      private static void method3() throws ClassNotFoundException, NoSuchFieldException {
          //        Field getField(String name): returns a single public member variable object
          //The member variable you want to get must be real
          //And must be public decorated
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect4.Student");
    
          //2. Get the member variable name
          //Field field = clazz.getField("name");
          //Field field = clazz.getField("name1");
          Field field = clazz.getField("money");
    
          //3. Print it
          System.out.println(field);
      }
    
      private static void method2() throws ClassNotFoundException {
          //        Field [] getdeclaraedfields(): returns an array of all member variable objects
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect4.Student");
    
          //2. Get all Field objects
          Field[] fields = clazz.getDeclaredFields();
    
          //3. Traversal
          for (Field field : fields) {
              System.out.println(field);
          }
      }
    
      private static void method1() throws ClassNotFoundException {
          //        Field[] getFields(): returns an array of all public member variable objects
    
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect4.Student");
    
          //2. Get the Field object
          Field[] fields = clazz.getFields();
    
          //3. Traversal
          for (Field field : fields) {
              System.out.println(field);
          }
      }
  }
  

2.4.2Field class is used to assign values to member variables

• Method introduction

  Method name	explain
  void set(Object obj, Object value)	assignment
  Object get(Object obj)	Get value

• Sample code

  // The Student class is the same as the previous example, which is not repeated here
  public class ReflectDemo2 {
      public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, IllegalAccessException, InstantiationException {
  //        Object get(Object obj) returns the value of the Field represented by the Field on the specified object.
          //method1();
          //method2();
  
      }
  
      private static void method2() throws ClassNotFoundException, NoSuchFieldException, InstantiationException, IllegalAccessException {
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect4.Student");
  
          //2. Get the object of the member variable Field
          Field field = clazz.getDeclaredField("money");
  
          //3. Cancel the access check
          field.setAccessible(true);
  
          //4. Call the get method to get the value
          //4.1 create an object
          Student student = (Student) clazz.newInstance();
          //4.2 get the value of money of the specified object
          Object o = field.get(student);
  
          //5. Print it
          System.out.println(o);
      }
  
      private static void method1() throws ClassNotFoundException, NoSuchFieldException, InstantiationException, IllegalAccessException {
          //        void set(Object obj, Object value): assign value to the member variable of obj object
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect4.Student");
  
          //2. Get the Field object name
          Field field = clazz.getField("name");
  
          //3. Use the set method for assignment
          //3.1 create a Student object first
          Student student = (Student) clazz.newInstance();
          //3.2 assignment can be made to the specified object only when there is an object
          field.set(student,"zhangsan");
  
          System.out.println(student);
      }
  }
  

2.5 reflection method to obtain members and use [application]

2.5.1 method for class class to obtain member method object

• Method classification

  Method name	explain
  Method[] getMethods()	Returns an array of all public member method objects, including inherited
  Method[] getDeclaredMethods()	Returns an array of all member method objects, excluding inherited
  Method getMethod(String name, Class<?>... parameterTypes)	Returns a single public member method object
  Method getDeclaredMethod(String name, Class<?>... parameterTypes)	Returns a single member method object

• Sample code

  public class Student {
  
      //Private, no parameters, no return value
      private void show() {
          System.out.println("Private show Method, no parameter, no return value");
      }
  
      //Public, no parameter, no return value
      public void function1() {
          System.out.println("function1 Method, no parameter, no return value");
      }
  
      //Public, with parameters and no return value
      public void function2(String name) {
          System.out.println("function2 Method with parameters and no return value,Parameter is" + name);
      }
  
      //Public, no parameter, return value
      public String function3() {
          System.out.println("function3 Method, no parameter, return value");
          return "aaa";
      }
  
      //Public, with parameters and return values
      public String function4(String name) {
          System.out.println("function4 Method with parameters and return values,Parameter is" + name);
          return "aaa";
      }
  }
  public class ReflectDemo1 {
      public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException {
          //method1();
          //method2();
          //method3();
          //method4();
          //method5();
      }
  
      private static void method5() throws ClassNotFoundException, NoSuchMethodException {
          //        Method getDeclaredMethod(String name, Class<?>... parameterTypes): 
  //                                Returns a single member method object
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect5.Student");
          //2. Get a member method show
          Method method = clazz.getDeclaredMethod("show");
          //3. Print it
          System.out.println(method);
      }
    
      private static void method4() throws ClassNotFoundException, NoSuchMethodException {
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect5.Student");
          //2. Get a method with formal parameters function2
          Method method = clazz.getMethod("function2", String.class);
          //3. Print it
          System.out.println(method);
      }
    
      private static void method3() throws ClassNotFoundException, NoSuchMethodException {
          //        Method getMethod(String name, Class<?>... parameterTypes) : 
  //                                Returns a single public member method object
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect5.Student");
          //2. Get member method function1
          Method method1 = clazz.getMethod("function1");
          //3. Print it
          System.out.println(method1);
      }
    
      private static void method2() throws ClassNotFoundException {
          //        Method[] getDeclaredMethods(): 
  //                                Returns an array of all member method objects, excluding inherited
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect5.Student");
    
          //2. Get Method object
          Method[] methods = clazz.getDeclaredMethods();
          //3. Traverse the array
          for (Method method : methods) {
              System.out.println(method);
          }
      }
    
      private static void method1() throws ClassNotFoundException {
          //        Method[] getMethods(): returns an array of all public member method objects, including inherited methods
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect5.Student");
          //2. Get member method object
          Method[] methods = clazz.getMethods();
          //3. Traversal
          for (Method method : methods) {
              System.out.println(method);
          }
      }
  }
  

2.5.2Method class is used to execute the method

• Method introduction

  Method name	explain
  Object invoke(Object obj, Object... args)	Operation method

  Parameter 1: call this method with obj object

  Parameter 2: the parameter passed by the calling method (if not, it will not be written)

  Return value: the return value of the method (if not, it will not be written)

• Sample code

  public class ReflectDemo2 {
      public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InstantiationException, InvocationTargetException {
  //        Object invoke(Object obj, Object... args): run the method
  //        Parameter 1: call this method with obj object
  //        Parameter 2: the parameter passed by the calling method (if not, it will not be written)
  //        Return value: the return value of the method (if not, it will not be written)
  
          //1. Get class object
          Class clazz = Class.forName("com.XXX.myreflect5.Student");
          //2. Get the Method object function4
          Method method = clazz.getMethod("function4", String.class);
          //3. Just run the function4 method
          //3.1 create a Student object as the caller of the method
          Student student = (Student) clazz.newInstance();
          //3.2 operation method
          Object result = method.invoke(student, "zhangsan");
          //4. Print the return value
          System.out.println(result);
      }
  }