Java running jar main class

Содержание

Running JAR-Packaged Software

Now that you have learned how to create JAR files, how do you actually run the code you packaged? Consider these scenarios:

Your JAR file contains an applet that is to be run inside a browser.
Your JAR file contains an application that is to be started from the command line.
Your JAR file contains code that you want to use as an extension.

This section will cover the first two situations. A separate trail in the tutorial on the extension mechanism covers the use of JAR files as extensions.

Applets Packaged in JAR Files

To start any applet from an HTML file for running inside a browser, you use the applet tag. For more information, see the Java Applets lesson. If the applet is bundled as a JAR file, the only thing you need to do differently is to use the archive parameter to specify the relative path to the JAR file.

As an example, use the TicTacToe demo applet. The applet tag in the HTML file that displays the applet can be marked up like this:

If the TicTacToe demo was packaged in a JAR file named TicTacToe.jar, you can modify the applet tag with the addition of an archive parameter:

The archive parameter specifies the relative path to the JAR file that contains TicTacToe.class. For this example it is assumed that the JAR file and the HTML file are in the same directory. If they are not, you must include the JAR file’s relative path in the archive parameter’s value. For example, if the JAR file was one directory below the HTML file in a directory called applets, the applet tag would look like this:

JAR Files as Applications

You can run JAR packaged applications with the Java launcher (java command). The basic command is:

The -jar flag tells the launcher that the application is packaged in the JAR file format. You can only specify one JAR file, which must contain all of the application-specific code.

Before you execute this command, make sure that the runtime environment has information about which class within the JAR file is the application’s entry point.

To indicate which class is the application’s entry point, you must add a Main-Class header to the JAR file’s manifest. The header takes the form:

The header’s value, classname, is the name of the class that is the application’s entry point.

For more information, see the Setting an Application’s Entry Point section.

When the Main-Class is set in the manifest file, you can run the application from the command line:

To run the application from the JAR file that is in another directory, you must specify the path of that directory: java -jar path/app.jar

Источник

Run a Java Application from the Command Line

As always, the writeup is super practical and based on a simple application that can work with documents with a mix of encrypted and unencrypted fields.

We rely on other people’s code in our own work. Every day.

It might be the language you’re writing in, the framework you’re building on, or some esoteric piece of software that does one thing so well you never found the need to implement it yourself.

The problem is, of course, when things fall apart in production — debugging the implementation of a 3rd party library you have no intimate knowledge of is, to say the least, tricky.

Lightrun is a new kind of debugger.

It’s one geared specifically towards real-life production environments. Using Lightrun, you can drill down into running applications, including 3rd party dependencies, with real-time logs, snapshots, and metrics.

Learn more in this quick, 5-minute Lightrun tutorial:

Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.

The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.

Critically, it has very minimal impact on your server’s performance, with most of the profiling work done separately — so it needs no server changes, agents or separate services.

Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:

DbSchema is a super-flexible database designer, which can take you from designing the DB with your team all the way to safely deploying the schema.

The way it does all of that is by using a design model, a database-independent image of the schema, which can be shared in a team using GIT and compared or deployed on to any database.

And, of course, it can be heavily visual, allowing you to interact with the database using diagrams, visually compose queries, explore the data, generate random data, import data or build HTML5 database reports.

**Get started with Spring 5 and Spring Boot 2, through the Learn Spring course:**

> CHECK OUT THE COURSE

1. Overview

Typically, every meaningful application includes one or more JAR files as dependencies. But there are times a JAR file itself represents a standalone application or a web application.

Here we’ll focus on the standalone application scenario. From now on, we’ll refer to it as a JAR application.

In this tutorial, we’ll first learn how to create a JAR application. Later, we’ll learn how to run a JAR application with or without command-line arguments.

Run JUnit Test Cases From the Command Line

Command-Line Arguments in Java

Command-Line Arguments in Spring Boot

2. Create a JAR Application

A JAR file can contain one or more main classes. Each main class is the entry point of an application. So, a JAR file can theoretically contain more than one application, but it has to contain at least one main class to be able to run.

A JAR file can have one entry point set in its manifest file. In this case, the JAR file is an executable JAR. The main class has to be included in that JAR file.

First, let’s see a quick example of how to compile our classes and create an executable JAR with a manifest file:

$ javac com/baeldung/jarArguments/*.java $ jar cfm JarExample.jar ../resources/example_manifest.txt com/baeldung/jarArguments/*.class

A nonexecutable JAR is simply a JAR file that doesn’t have a Main-Class defined in the manifest file. As we’ll see later, we can still run a main class that’s contained in the JAR file itself.

Here’s how we would create a nonexecutable JAR without a manifest file:

$ jar cf JarExample2.jar com/baeldung/jarArguments/*.class

3. Java Command-Line Arguments

Just like any application, a JAR application accepts any number of arguments, including zero arguments. It all depends on the application’s need.

This allows the user to specify configuration information when the application is launched.

As a result, the application can avoid hard-coded values, and it still can handle many different use cases.

An argument can contain any alphanumeric characters, unicode characters and possibly some special characters allowed by the shell, for example, @.

Arguments are separated by one or more spaces. If an argument needs to contain spaces, the spaces have to be enclosed between quotes. Either single quotes or double quotes work fine.

Usually, for a typical Java application, when invoking the application, the user enters command-line arguments after the name of the class.

However, that’s not always the case for JAR applications.

As we discussed, the entry point of a Java main class is the main method. The arguments are all Strings and are passed to the main method as a String array.

That said, inside the application, we can convert any element of the String array to other data types, such as char, int, double, their wrapper classes or other appropriate types.

4. Run an Executable JAR with Arguments

Let’s see the basic syntax for running an executable JAR file with arguments:

java -jar jar-file-name [args …]

The executable JAR created earlier is a simple application that just prints out the arguments passed in. We can run it with any number of arguments.

Here’s an example with two arguments:

We’ll see this output in the console:

So, when invoking an executable JAR, we don’t need to specify the main class name on the command line. We simply add our arguments after the JAR file name. If we do provide a class name after the executable JAR file name, it simply becomes the first argument to the actual main class.

Most times, a JAR application is an executable JAR. An executable JAR can have a maximum of one main class defined in the manifest file.

Consequently, other applications in the same executable JAR file can’t be set in the manifest file, but we can still run them from the command line just like we would for a nonexecutable JAR. We’ll see exactly how in the next section.

5. Run a Nonexecutable JAR with Arguments

To run an application in a nonexecutable JAR file, we have to use -cp option instead of -jar.

We’ll use the -cp option (short for classpath) to specify the JAR file that contains the class file we want to execute:

java -cp jar-file-name main-class-name [args …]

As we can see, in this case, we’ll have to include the main class name in the command line, followed by arguments.

The nonexecutable JAR created earlier contains the same simple application. We can run it with any (including zero) arguments.