imshouvikb@gmail.com/imshouvikb@gmail.com

spring-core/src/main/java/com/baeldung/app/DependencyInjectionApp.java

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+package com.baeldung.app;
+
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+
+import com.baeldung.configuration.DependencyInjectionSpring;
+import com.baeldung.domain.Course;
+import com.baeldung.domain.Department;
+import com.baeldung.domain.Employee;
+import com.baeldung.domain.Student;
+
+public class DependencyInjectionApp {
+
+    public static void main(String[] args) {
+
+        ApplicationContext context = new AnnotationConfigApplicationContext(DependencyInjectionSpring.class);
+
+        System.out.println("Mandatory Dependency");
+        Employee employee = context.getBean(Employee.class);
+        employee.setId(1);
+        employee.setName("Robert Langdon");
+        Department department = context.getBean(Department.class);
+        System.out.println("Department Name: "+department.getName());
+        System.out.println("Employee in department: "+department.getEmployee().getName());
+
+        System.out.println("Optional Dependency");
+        Student student = context.getBean(Student.class);
+        student.setId(1);
+        student.setName("Albert Einstein");
+        Course course = context.getBean(Course.class);
+        System.out.println("Course:"+course.getName());
+        System.out.println("Student enrolled: "+course.getStudent().getName());
+        ((ConfigurableApplicationContext)context).close();
+    }
+
+}

spring-core/src/main/java/com/baeldung/configuration/DependencyInjectionSpring.java

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+package com.baeldung.configuration;
+
+import org.springframework.context.annotation.ComponentScan;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration
+@ComponentScan("com.baeldung.domain")
+public class DependencyInjectionSpring {}

spring-core/src/main/java/com/baeldung/domain/Course.java

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+package com.baeldung.domain;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.stereotype.Component;
+
+@Component
+public class Course {
+    private int id;
+    private String name = "Physics";
+
+    /* Dependency/collaborator */
+    private Student student;
+
+    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 Student getStudent() {
+        return student;
+    }
+
+    /* 
+     * Optional dependency since a course may or 
+     * may not have student enrolled into it 
+     */
+    @Autowired(required = false)
+    public void setStudent(Student student) {
+        this.student = student;
+    }
+}

spring-core/src/main/java/com/baeldung/domain/Department.java

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+package com.baeldung.domain;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.stereotype.Component;
+
+@Component
+public class Department {
+
+    private String name = "Technology";
+
+    /* Dependency/collaborator */
+    private Employee employee;
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public Employee getEmployee() {
+        return employee;
+    }
+
+    /* Mandatory dependency since a
+     * department will have an employee.
+     */
+    @Autowired
+    public Department(Employee employee) {
+        this.employee = employee;
+    }
+
+}

spring-core/src/main/java/com/baeldung/domain/Employee.java

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+package com.baeldung.domain;
+
+import org.springframework.stereotype.Component;
+
+@Component
+public class Employee {
+
+    private int id = 1;
+    private String name = "Robert Langdon";
+
+    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;
+    }
+
+}

spring-core/src/main/java/com/baeldung/domain/Student.java

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+package com.baeldung.domain;
+
+import org.springframework.stereotype.Component;
+
+@Component
+public class Student {
+    private int id = 1;
+    private String name = "Albert Einstein";
+
+    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;
+    }
+}

spring-core/src/test/java/com/baeldung/dependencyinjectiontypes/WhenUsingConstructorInjection.java

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+package com.baeldung.dependencyinjectiontypes;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.test.context.ContextConfiguration;
+import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
+
+import com.baeldung.configuration.DependencyInjectionSpring;
+import com.baeldung.domain.Department;
+
+@RunWith(SpringJUnit4ClassRunner.class)
+@ContextConfiguration(classes = DependencyInjectionSpring.class)
+public class WhenUsingConstructorInjection {
+
+    @Autowired
+    private Department department;
+
+    @Test
+    public void shouldReturnDepartmentName() {
+        Assert.assertEquals("Technology", department.getName());
+    }
+
+    @Test
+    public void shouldReturnEmployeeName() {
+        Assert.assertEquals("Robert Langdon", department.getEmployee()
+            .getName());
+    }
+}

spring-core/src/test/java/com/baeldung/dependencyinjectiontypes/WhenUsingSetterInjection.java

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+package com.baeldung.dependencyinjectiontypes;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.test.context.ContextConfiguration;
+import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
+
+import com.baeldung.configuration.DependencyInjectionSpring;
+import com.baeldung.domain.Course;
+
+@RunWith(SpringJUnit4ClassRunner.class)
+@ContextConfiguration(classes = DependencyInjectionSpring.class)
+public class WhenUsingSetterInjection {
+
+    @Autowired
+    private Course course;
+
+    @Test
+    public void shouldReturnCourseName() {
+        Assert.assertEquals("Physics", course.getName());
+    }
+
+    @Test
+    public void shouldReturnStudentName() {
+        Assert.assertEquals("Albert Einstein", course.getStudent()
+            .getName());
+    }
+}