◀Table of Contents

Getting Started

Native Image is a technology to compile Java code ahead-of-time to a binary – a native executable. A native executable includes only the code required at run time, that is the application classes, standard-library classes, the language runtime, and statically-linked native code from the JDK.

An executable file produced by Native Image has several important advantages, in that it

Uses a fraction of the resources required by the Java Virtual Machine, so is cheaper to run
Starts in milliseconds
Delivers peak performance immediately, with no warmup
Can be packaged into a lightweight container image for fast and efficient deployment
Presents a reduced attack surface

A native executable is created by the Native Image builder or native-image that processes your application classes and other metadata to create a binary for a specific operating system and architecture. First, the native-image tool performs static analysis of your code to determine the classes and methods that are reachable when your application runs. Second, it compiles classes, methods, and resources into a binary. This entire process is called build time to clearly distinguish it from the compilation of Java source code to bytecode.

The native-image tool can be used to build a native executable, which is the default, or a native shared library. This quick start guide focuses on building a native executable; to learn more about native shared libraries, go here.

To get used to Native Image terminology and get better understanding of the technology, we recommend you to read the Basics of Native Image.

Install Native Image
Build a Native Executable
Configuring Native Image with Third-Party Libraries
License
Further Reading

Install Native Image

Native Image can be added to GraalVM with the GraalVM Updater tool.

Run this command to install Native Image:

gu install native-image

The native-image tool is installed in the $JAVA_HOME/bin directory.

Prerequisites

The native-image tool depends on the local toolchain (header files for the C library, glibc-devel, zlib, gcc, and/or libstdc++-static). These dependencies can be installed (if not yet installed) using a package manager on your machine. Choose your operating system to find instructions to meet the prerequisites.

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Build a Native Executable

The native-image tool takes Java bytecode as its input. You can build a native executable from a class file, from a JAR file, or from a module (with Java 9 and higher).

From a Class

To build a native executable from a Java class file in the current working directory, use the following command:

native-image [options] class [imagename] [options]

For example, build a native executable for a HelloWorld application.

Save this code into file named HelloWorld.java:

 public class HelloWorld {
     public static void main(String[] args) {
         System.out.println("Hello, Native World!");
     }
 }

Compile it and build a native executable from the Java class:
```
 javac HelloWorld.java
 native-image HelloWorld
```
It will create a native executable, helloWorld, in the current working directory.

Run the application:

 ./helloWorld

You can time it to see the resources used:

 time -f 'Elapsed Time: %e s Max RSS: %M KB' ./helloworld
 # Hello, Native World!
 # Elapsed Time: 0.00 s Max RSS: 7620 KB

From a JAR file

To build a native executable from a JAR file in the current working directory, use the following command:

native-image [options] -jar jarfile [imagename]

Prepare the application.

Create a new Java project named “App”, for example in your favorite IDE or from your terminal, with the following structure:
```
  | src
  |   --com/
  |      -- example
  |          -- App.java
```

Add the following Java code into the src/com/example/App.java file:

  package com.example;

  public class App {

      public static void main(String[] args) {
          String str = "Native Image is awesome";
          String reversed = reverseString(str);
          System.out.println("The reversed string is: " + reversed);
      }

      public static String reverseString(String str) {
          if (str.isEmpty())
              return str;
          return reverseString(str.substring(1)) + str.charAt(0);
      }
  }

This is a small Java application that reverses a String using recursion.

Then create a text file in the root directory named META-INF/MANIFEST.MF which contains the following:
```
  Manifest-Version: 1.0
  Main-Class: com.example.App
```

Compile the application:
```
 javac -d build src/com/example/App.java
```
This produces the file App.class in the build/com/example directory.
Create a runnable JAR file by including the manifest file:
```
 jar cfvm App.jar META-INF/MANIFEST.MF -C build . 
```
It will generate a runnable JAR file, named App.jar, in the root directory: To view its contents, type jar tf App.jar.
Create a native executable from that JAR file:
```
 native-image -jar App.jar
```
It will produce a native executable in the project root directory.
Run the native executable:
```
 ./App
```

The native-image tool can provide the class path for all classes using the familiar option from the java launcher: -cp, followed by a list of directories or JAR files, separated by : on Linux and macOS platforms, or ; on Windows. The name of the class containing the main method is the last argument, or you can use the -jar option and provide a JAR file that specifies the main method in its manifest.

From a Module

You can also convert a modularized Java application into a native executable.

The command to build a native executable from a Java module is:

native-image [options] --module <module>[/<mainclass>] [options]

For more information about how to produce a native executable from a modular Java application, see Building a HelloWorld Java Module into a Native Executable.

Build Overview

There many options you can pass to the native-image builder to configure the image build process. Run native-image --help to see the full list. The options passed to native-image are evaluated left-to-right.

For different image build tweaks and to learn more about build time configuration, see Native Image Build Configuration.

Native Image will output the progress and various statistics during the build. To learn more about the output and the different build phases, see Build Output.

Configuring Native Image with Third-Party Libraries

For more complex applications that use external libraries, you must provide the native-image builder with metadata.

Building a standalone binary with the native-image tool takes place under a “closed world assumption”. The native-image tool performs an analysis to see which classes, methods, and fields within your application are reachable and must be included in the native image. The analysis is static: it does not run your application. This means that all the bytecode in your application that can be called at run time must be known (observed and analyzed) at build time.

The analysis can determine some cases of dynamic class loading, but it cannot always exhaustively predict all usages of the Java Native Interface (JNI), Java Reflection, Dynamic Proxy objects, or class path resources. To deal with these dynamic features of Java, you inform the analysis with details of the classes that use Reflection, Proxy, and so on, or what classes to be dynamically loaded. To achieve this, you either provide the native-image tool with JSON-formatted configuration files or pre-compute metadata in the code.

To learn more about metadata, ways to provide it, and supported metadata types, see Reachability Metadata. To automatically collect metadata for your application, see Automatic Collection of Metadata.

There are also Maven and Gradle plugins for Native Image to automate building, testing and configuring native executables. Learn more here.

Lastly, not all applications may be compatible with Native Image. For more details, see Native Image Compatibility Guide.

Native Image can also interop with native languages through a custom API. Using this API, you can specify custom native entry points into your Java application and build it into a nativw shared library. To learn more, see Interoperability with Native Code.

License

The Native Image technology is distributed as a separate installable to GraalVM. Native Image for GraalVM Community Edition is licensed under the GPL 2 with Classpath Exception.

Native Image for GraalVM Enterprise Edition is licensed under the Oracle Technology Network License Agreement for GraalVM Enterprise Edition.