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Native Image

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.

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

Prerequisites

The native-image tool, available in the bin directory of your GraalVM installation, 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]

The default behavior of native-image is aligned with the java command which means you can pass the -jar, -cp, -m options to build with Native Image as you would normally do with java. For example, java -jar App.jar someArgument becomes native-image -jar App.jar and ./App someArgument.

Follow this guide to build a native executable from a JAR file.

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.

Getting Notified When the Build Process Is Done

Depending on the size of your application and the available resources of your build machine, it can take a few minutes to AOT-compile your Java application to a native executable. If you are building your project in the background, consider using a command that notifies you when the build process is completed. Below, example commands are listed per operating system:

Linux

# Ring the terminal bell
native-image -jar App.jar ... ; printf '\a'

# Use libnotify to create a desktop notification
native-image -jar App.jar ... ; notify-send "GraalVM Native Image build completed with exit code $?"

# Use Zenity to open an info dialog box with text
native-image -jar App.jar ... ; zenity --info --text="GraalVM Native Image build completed with exit code $?"

macOS

# Ring the terminal bell
native-image -jar App.jar ... ; printf '\a'

# Use Speech Synthesis
native-image -jar App.jar ... ; say "GraalVM Native Image build completed"

Windows

REM Ring the terminal bell (press Ctrl+G to enter ^G)
native-image.exe -jar App.jar & echo ^G

REM Open an info dialog box with text
native-image.exe -jar App.jar & msg "%username%" GraalVM Native Image build completed

Build Configuration

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

For different 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.

Native Image and 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.

Some applications may need additional configuration to be compiled with GraalVM 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.