The Java reflection mechanism refers to the mechanism that dynamically obtains class information and operates on it during program execution. Through reflection mechanism, we can check or modify information such as classes, methods, and properties at runtime, and even create objects and call methods at runtime. The core of the Java reflection mechanism is the classes provided by the java. lang. reflect package, with the most important being the Class class. The Class class represents the entire class information of a class, including class name, parent class, methods, properties, etc. We can access this information and manipulate it through objects of the Class class. The following is an example code of a simple Java reflection mechanism: ``` import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.Method; public class ReflectionDemo { public static void main(String[] args) throws Exception { //Get the Class object of the Person class Class<Person> personClass = Person.class; //Get Class Name String className = personClass.getName(); System.out.println("Class Name: " + className); //Obtain the construction method of the class Constructor<Person> constructor = personClass.getConstructor(String.class, int.class); System.out.println("Constructor: " + constructor); //Creating Objects Person person = constructor.newInstance("John", 28); System.out.println("Person: " + person); //Method for obtaining classes Method sayHelloMethod = personClass.getMethod("sayHello"); System.out.println("Method: " + sayHelloMethod); //Calling methods sayHelloMethod.invoke(person); //Get the properties of the class Field nameField = personClass.getDeclaredField("name"); System.out.println("Field: " + nameField); //Modify attribute values nameField.setAccessible(true); nameField.set(person, "Tom"); //Call the method to verify if the property value modification was successful sayHelloMethod.invoke(person); } } ``` In the above example code, the reflection mechanism can dynamically obtain information about the Person class, including class name, construction method, method, attribute, etc., and operate on it. The final output result is: ``` Class Name: Person Constructor: public Person(java.lang.String,int) Person: Person [name=John, age=28] Method: public void Person.sayHello() Hello, John// Calling the sayHello method to output the result Hello, Tom// After modifying the name attribute value, call the sayHello method again to output the result ``` Summary: The Java reflection mechanism provides the ability to dynamically obtain class information and manipulate it at runtime. Through the reflection mechanism, flexible and dynamic code writing can be achieved, making the program more scalable and versatile. Although the reflection mechanism is powerful, it also increases the complexity of the code and incurs certain performance overhead. Therefore, caution should be exercised when using it to avoid abuse.

The application of Java reflection in unit testing is mainly reflected in dynamically obtaining class information and calling class methods through reflection mechanisms, thereby achieving the testing of class properties and functions. Below is a simple example code to illustrate the application of Java reflection in unit testing: ```java //Tested classes public class Calculator { private int add(int a, int b) { return a + b; } private int subtract(int a, int b) { return a - b; } } //Testing class public class CalculatorTest { public static void main(String[] args) throws Exception { //Create an instance of the tested class Calculator calculator = new Calculator(); //Method of obtaining classes through reflection Method addMethod = Calculator.class.getDeclaredMethod("add", int.class, int.class); Method subtractMethod = Calculator.class.getDeclaredMethod("subtract", int.class, int.class); //Accessibility of setting methods addMethod.setAccessible(true); subtractMethod.setAccessible(true); //Test add method int result1 = (int) addMethod.invoke(calculator, 2, 3); System. out. println (result1)// Output 5 //Test Subtract Method int result2 = (int) subtractMethod.invoke(calculator, 5, 3); System. out. println (result2)// Output 2 } } ``` Summary: The application of Java reflection in unit testing can dynamically obtain class information and call class methods for testing. By using reflection, we can test private methods without accessing them, which is very useful in some special situations. However, due to the fact that reflection involves dynamic method calls, its performance is relatively low, so frequent use of reflection should be avoided in practical development as much as possible.

There are many cases of using reflection in the ORM framework, and the following are a few common cases, using the Java language as an example, and providing corresponding code examples: 1. Mapping relationship between entities and database tables: ```java @Table(name = "user") public class User { @Column(name = "id", type = "int") private int id; @Column(name = "name", type = "varchar") private String name; //The getter and setter methods are omitted } public class TableUtils { public static String getTableName(Class<?> clazz) { Table table = clazz.getAnnotation(Table.class); if (table != null) { return table.name(); } return null; } public static Map<String, String> getColumnNames(Class<?> clazz) { Map<String, String> columnNames = new HashMap<>(); Field[] fields = clazz.getDeclaredFields(); for (Field field : fields) { Column column = field.getAnnotation(Column.class); if (column != null) { columnNames.put(field.getName(), column.name()); } } return columnNames; } } ``` 2. Automatically generate SQL statements based on entity classes: ```java public class SqlBuilder { public static String buildInsertStatement(Object obj) { Class<?> clazz = obj.getClass(); String tableName = TableUtils.getTableName(clazz); Map<String, String> columnNames = TableUtils.getColumnNames(clazz); StringBuilder sb = new StringBuilder(); sb.append("INSERT INTO ").append(tableName).append(" ("); Field[] fields = clazz.getDeclaredFields(); for (Field field : fields) { String columnName = columnNames.get(field.getName()); if (columnName != null) { sb.append(columnName).append(", "); } } sb.delete(sb.length() - 2, sb.length()); sb.append(") VALUES ("); for (Field field : fields) { String columnName = columnNames.get(field.getName()); if (columnName != null) { Object value = field.get(obj); sb.append("'").append(value).append("', "); } } sb.delete(sb.length() - 2, sb.length()); sb.append(")"); return sb.toString(); } } ``` Summary: Using reflection in the ORM framework can easily generate corresponding database statements based on entity classes, simplifying the workload of developers. Using reflection can dynamically obtain attribute information of entity classes and process it based on metadata such as annotations, thereby achieving mapping relationships with database tables. In addition, in the actual use process, it is necessary to pay attention to the potential performance losses caused by reflection operations and optimize the code reasonably.