Deep understanding of Jar package structure

Article directory

      • **Basic concepts of Jar packages**
      • **Advantages of Jar package**
      • **Creation and use of Jar packages**
      • **Structure of Jar package**
      • **The far-reaching impact of Jar packages**
      • **Spring Boot Jar package structure example**
        • java code example
      • Example of Jar internal structure file:
        • **1. Manifest file (MANIFEST.MF) example**
        • **2. Spring Boot main class example**
        • **3. Static resource example**
        • **4. Application Configuration Example**
        • **5. Java class file example**
        • **6. Jar package dependency example**
        • **7. Module description file example (in Java 9 and later)**
      • **Summarize**

Basic concepts of Jar packages

First, let’s start with the basic concepts of JAR packages. JAR, the full name of Java Archive, is a compressed file format of Java. It is mainly used to package and distribute Java classes and related resources, including Java class files, resource files, configuration files and other Java application-related files. Unlike compiling and running Java source code directly, using JAR packages makes it easier to deploy and distribute applications.

Advantages of Jar packages

Next, we will discuss the advantages of using JAR packages. First of all, JAR packages can combine multiple Java classes and related resources to facilitate developers to manage and deploy applications. JAR packages simplify the application deployment and distribution process and reduce management complexity by packaging various parts of an application into a single file.

Secondly, JAR packages can significantly reduce the size of the application. Because the JAR package has the function of removing redundant files and directory structures, the size of the application can be compressed to a minimum. This is particularly important for distributing and deploying applications, especially with significant advantages in network transmission and storage.

In addition, JAR packages provide better security and reliability. They can contain digital signatures to verify their identity and integrity, ensuring the application’s secure operation. Through digital signatures, we can confirm the source and content of the JAR package and avoid potential security risks.

Creation and use of Jar packages

So, how to create and use JAR packages?

To create a JAR package, we can use the “jar” command line tool from the Java Development Kit (JDK).

JAR files can be created, updated and listed by executing specific commands and parameters. In addition, JAR packages can be easily created in an integrated development environment (IDE). The specific steps depend on the IDE used. Most IDEs provide corresponding wizards and tools to simplify the creation process of JAR packages.

For example, the following uses maven to create a jar package:

Once the JAR package is created, we can use it as part of a Java application. A JAR package can be added to a Java application’s classpath so that its classes and methods can be used at runtime. In Java code, we can use the import statement to import classes and methods in the JAR package and then use them in the application. By adding a JAR package to the classpath, we can use its classes and methods in our application without having to worry about path and dependency management.

Jar package structure

Spring Boot’s Jar package follows the general Jar package structure, but it also contains Spring Boot-specific elements.

The following is the structure of the Spring Boot Jar package:

  1. META-INF directory: Like the traditional Jar package, Spring Boot’s Jar package contains the META-INF directory. The most important file is the manifest file (Manifest.mf), which contains the application metadata information.

  2. BOOT-INF directory: This is a Spring Boot-specific directory that contains all the class files and resources of the application. It is further divided into subdirectories:

    • classes directory: Contains compiled Java class files.
    • lib directory: Contains all dependent Jar packages that are referenced by Spring Boot applications.

  3. Static resource directories (such as templates and static): These directories contain the static resource files of the application, such as HTML, CSS, JavaScript, etc.

  4. application.properties or application.yml files: These files contain the configuration information of the application, such as database connection, port number, log level, etc.

The far-reaching impact of Jar packages

Spring Boot’s Jar package structure has a profound impact on the development and deployment of Spring Boot applications:

  1. Executable Jar package: Spring Boot’s Jar package is executable, which means we can run Spring Boot applications directly without an external web server. This simplifies the deployment and operation process.

  2. Embedded Servlet container: Spring Boot’s Jar package contains an embedded Servlet container, such as Tomcat or Jetty. This allows applications to run without the need for an external Servlet container.

  3. Dependency management: Spring Boot’s Jar package structure allows us to package all dependencies into a Jar to reduce the complexity of dependency management. This ensures that the application is compatible with a specific version of the library.

  4. Automatic configuration: Spring Boot provides automatic configuration, which can automatically configure many functions based on application dependencies, thus simplifying the development process.

  5. Externalized configuration: Spring Boot’s Jar package allows the use of external configuration files, which means that we can modify the application’s configuration without repackaging the Jar package.

When it comes to explaining and exemplifying the Jar package structure, we will consider the Jar package structure of a typical Spring Boot application and provide corresponding code examples:

Spring Boot Jar package structure example

Let’s first look at a simplified example of the Jar package structure of a Spring Boot application:

my-spring-boot-app.jar
│
├── META-INF/
│ └── MANIFEST.MF
│
├── BOOT-INF/
│ ├── classes/
│ │ ├── com/
│ │ │ └── example/
│ │ │ └── MySpringBootApplication.class
│ │ └── application.properties
│
│ ├── lib/
│ │ ├── spring-boot-2.6.0.jar
│ │ ├── spring-core-5.3.0.jar
│ │ ├── ...
│ │
│ └── ...
│
└── static/
    ├── index.html
    └── style.css
java code example

Now, let’s provide some code examples based on this structure:

1. Spring Boot application main class

In the MySpringBootApplication class, we usually have a main entry point for a Spring Boot application, as shown below:

package com.example;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MySpringBootApplication {<!-- -->
    public static void main(String[] args) {<!-- -->
        SpringApplication.run(MySpringBootApplication.class, args);
    }
}

2. Static resources

In the static directory, we can place static resource files, such as index.html and style.css. These files can be accessed via HTTP like:

  • http://localhost:8080/index.html
  • http://localhost:8080/style.css

3. Application configuration

The application.properties file contains the configuration information of the application. For example, we can define the port number and data source configuration like:

server.port=8080
spring.datasource.url=jdbc:mysql://localhost/mydatabase

These are a simplified example showing the Spring Boot Jar package structure and the main Java classes and configuration files.

Please note that an actual Spring Boot application may be more complex and contain many more classes and resource files, depending on the application’s functionality and needs.

Hopefully these code examples help to better understand Spring Boot’s Jar package structure. If we need more detailed examples or have other specific questions, please feel free to ask.

Example of Jar internal structure file:

1. Manifest file (MANIFEST.MF) example

The manifest file is a text file usually contained in the META-INF directory. It contains metadata information about the application, such as version, main class, etc. Here is a simple manifest file example:

Manifest-Version: 1.0
Main-Class: com.example.MySpringBootApplication

In this example, Main-Class specifies the main class of the application, which is the entry point of the executable Jar package.

2. Spring Boot main class example

The main class is the entry point of a Spring Boot application. Here is the code for an example main class:

package com.example;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MySpringBootApplication {<!-- -->
    public static void main(String[] args) {<!-- -->
        SpringApplication.run(MySpringBootApplication.class, args);
    }
}

This is a typical Spring Boot main class, using the @SpringBootApplication annotation to identify it as the entry point of the Spring Boot application.

3. Static resource example

In the static directory, we can place static resource files. The following is a sample static resource file:

  • index.htmlExample:
<!DOCTYPE html>
<html>
<head>
    <title>My Spring Boot App</title>
</head>
<body>
    <h1>Hello, Spring Boot!</h1>
</body>
</html>
  • style.cssExample:
/* style.css */
h1 {<!-- -->
    color: blue;
}

These sample static resource files can be used to build simple web pages.

4. Application configuration example

Application configuration is usually contained in the application.properties file. The following is an example configuration:

# Set port number
server.port=8080

# Database configuration
spring.datasource.url=jdbc:mysql://localhost/mydatabase
spring.datasource.username=myuser
spring.datasource.password=mypassword

This is a simple configuration example that sets the port number and database connection information.

These examples represent different file types in Spring Boot jars, from manifest files, main classes, static resources to application configurations. Together, these files form the Jar package structure of a typical Spring Boot application. If we need more examples or have other specific questions, please feel free to ask.

Let’s continue with sample code as we dive deeper into the Spring Boot Jar package structure, including more types of files and elements.

5. Java class file example

The BOOT-INF/classes directory contains the Java class files of the application. The following is a sample Java class, commonly used in controllers:

package com.example.controller;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloController {<!-- -->

    @GetMapping("/hello")
    public String sayHello() {<!-- -->
        return "Hello, Spring Boot!";
    }
}

This is a simple Spring Boot controller that responds to GET requests and returns a hello message.

6. Jar package dependency example

The BOOT-INF/lib directory contains the dependent Jar packages of the application. The following are some examples of dependent Jar packages, and there may be more actual dependencies:

  • spring-boot-2.6.0.jar
  • spring-core-5.3.0.jar

These Jar packages are the core dependencies of Spring Boot applications, and they contain the core functionality of the Spring framework.

7. Module description file example (in Java 9 and later)

In Java 9 and later versions, Jar packages can contain module description files. Here is an example of a simple module description file:

module com.example {
    requires spring.core;
    exports com.example;
}

This module description file describes the module’s dependencies and exported packages.

These examples show the various files and elements in the Spring Boot Jar package, including Java class files, dependent Jar packages, and module description files that may exist in Java 9 and later versions. These elements together form the Jar package structure of a complete Spring Boot application. If we need more examples or have other specific questions, please feel free to ask.

Summary

Spring Boot’s Jar package structure is a key component of a Spring Boot application. It allows the creation of self-contained, executable applications while providing built-in servlet containers, dependency management, automatic configuration and externalized configuration. Understanding Spring Boot’s Jar package structure is very important for the development and deployment of Spring Boot applications. It simplifies the development process while providing a better deployment and running experience.