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1. Introduction

In this article, we’ll look at the Java Ahead of Time (AOT) Compiler, which is described in JEP-295 and was added as an experimental feature in Java 9.

First, we’ll see what AOT is, and second, we’ll look at a simple example. Third, we’ll see some restrictions of AOT, and lastly, we’ll discuss some possible use cases.

2. What Is Ahead of Time Compilation?

AOT compilation is one way of improving the performance of Java programs and in particular the startup time of the JVM. The JVM executes Java bytecode and compiles frequently executed code to native code. This is called Just-in-Time (JIT) Compilation. The JVM decides which code to JIT compile based on profiling information collected during execution.

While this technique enables the JVM to produce highly optimized code and improves peak performance, the startup time is likely not optimal, as the executed code is not yet JIT compiled. AOT aims to improve this so-called warming-up period. The compiler used for AOT is Graal.

In this article, we won’t look at JIT and Graal in detail. Please refer to our other articles for an overview of performance improvements in Java 9 and 10, as well as a deep dive into the Graal JIT Compiler.

3. Example

For this example, we’ll use a very simple class, compile it, and see how to use the resulting library.

3.1. AOT Compilation

Let’s take a quick look at our sample class:

public class JaotCompilation {

    public static void main(String[] argv) {

    public static String message() {
        return "The JAOT compiler says 'Hello'";

Before we can use the AOT compiler, we need to compile the class with the Java compiler:


We then pass the resulting JaotCompilation.class to the AOT compiler, which is located in the same directory as the standard Java compiler:

jaotc --output JaotCompilation.class

This produces the library in the current directory.

3.2. Running the Program

We can then execute the program:

java -XX:AOTLibrary=./ JaotCompilation

The argument -XX:AOTLibrary accepts a relative or full path to the library. Alternatively, we can copy the library to the lib folder in the Java home directory and only pass the name of the library.

3.3. Verifying That the Library Is Called and Used

We can see that the library was indeed loaded by adding -XX:+PrintAOT as a JVM argument:

java -XX:+PrintAOT -XX:AOTLibrary=./ JaotCompilation

The output will look like:

77    1     loaded    ./  aot library

However, this only tells us that the library was loaded, but not that it was actually used. By passing the argument -verbose, we can see that the methods in the library are indeed called:

java -XX:AOTLibrary=./ -verbose -XX:+PrintAOT JaotCompilation 

The output will contain the lines:

11    1     loaded    ./  aot library
116    1     aot[ 1]   jaotc.JaotCompilation.<init>()V
116    2     aot[ 1]   jaotc.JaotCompilation.message()Ljava/lang/String;
116    3     aot[ 1]   jaotc.JaotCompilation.main([Ljava/lang/String;)V
The JAOT compiler says 'Hello'

The AOT compiled library contains a class fingerprint, which must match the fingerprint of the .class file.

Let’s change the code in the class to return a different message:

public static String message() {
    return "The JAOT compiler says 'Good morning'";

If we execute the program without AOT compiling the modified class:

java -XX:AOTLibrary=./ -verbose -XX:+PrintAOT JaotCompilation

Then the output will contain only:

 11 1 loaded ./ aot library
The JAOT compiler says 'Good morning'

We can see that the methods in the library won’t be called, as the bytecode of the class has changed. The idea behind this is that the program will always produce the same result, no matter if an AOT compiled library is loaded or not.

4. More AOT and JVM Arguments

4.1. AOT Compilation of Java Modules

It’s also possible to AOT compile a module:

jaotc --output --module java.base

The resulting library is about 320 MB in size and takes some time to load. The size can be reduced by selecting the packages and classes to be AOT compiled.

We’ll look at how to do that below, however, we’ll not dive deeply into all the details.

4.2. Selective Compilation with Compile Commands

To prevent the AOT compiled library of a Java module from becoming too large, we can add compile commands to limit the scope of what gets AOT compiled. These commands need to be in a text file – in our example, we’ll use the file complileCommands.txt:

compileOnly java.lang.*

Then, we add it to the compile command:

jaotc --output --module java.base --compile-commands compileCommands.txt

The resulting library will only contain the AOT compiled classes in the package java.lang.

To gain real performance improvement, we need to find out which classes are invoked during the warm-up of the JVM.

This can be achieved by adding several JVM arguments:

java -XX:+UnlockDiagnosticVMOptions -XX:+LogTouchedMethods -XX:+PrintTouchedMethodsAtExit JaotCompilation

In this article, we won’t dive deeper into this technique.

4.3. AOT Compilation of a Single Class

We can compile a single class with the argument –class-name:

jaotc --output --class-name java.lang.String

The resulting library will only contain the class String.

4.4. Compile for Tiered

By default, the AOT compiled code will always be used, and no JIT compilation will happen for the classes included in the library. If we want to include the profiling information in the library, we can add the argument compile-for-tiered:

jaotc --output --compile-for-tiered JaotCompilation.class

The pre-compiled code in the library will be used until the bytecode becomes eligible for JIT compilation.

5. Possible Use Cases for AOT Compilation

One use case for AOT is short running programs, which finish execution before any JIT compilation occurs.

Another use case is embedded environments, where JIT isn’t possible.

At this point, we also need to note that the AOT compiled library can only be loaded from a Java class with identical bytecode, thus it cannot be loaded via JNI.

6. AOT and Amazon Lambda

A possible use case for AOT-compiled code is short-lived lambda functions where short startup time is important. In this section, we’ll look at how we can run AOT compiled Java code on AWS Lambda.

Using AOT compilation with AWS Lambda requires the library to be built on an operating system that is compatible with the operating system used on AWS. At the time of writing, this is Amazon Linux 2.

Furthermore, the Java version needs to match. AWS provides the Amazon Corretto Java 11 JVM. In order to have an environment to compile our library, we’ll install Amazon Linux 2 and Amazon Corretto in Docker.

We won’t discuss all the details of using Docker and AWS Lambda but only outline the most important steps. For more information on how to use Docker, please refer to its official documentation here.

For more details about creating a Lambda function with Java, you can have a look at our article AWS Lambda With Java.

6.1. Configuration of Our Development Environment

First, we need to pull the Docker image for Amazon Linux 2 and install Amazon Corretto:

# download Amazon Linux 
docker pull amazonlinux 

# inside the Docker container, install Amazon Corretto
yum install java-11-amazon-corretto

# some additional libraries needed for jaotc
yum install binutils.x86_64

6.2. Compile the Class and Library

Inside our Docker container, we execute the following commands:

# create folder aot
mkdir aot
cd aot
mkdir jaotc
cd jaotc

The name of the folder is only an example and can, of course, be any other name.

package jaotc;

public class JaotCompilation {
    public static int message(int input) {
        return input * 2;

The next step is to compile the class and library:

cd ..
jaotc -J-XX:+UseSerialGC --output jaotc/JaotCompilation.class

Here, it’s important to use the same garbage collector as is used on AWS. If our library cannot be loaded on AWS Lambda, we might want to check which garbage collector is actually used with the following command:

java -XX:+PrintCommandLineFlags -version

Now, we can create a zip file that contains our library and class file:

zip -r jaotc/

6.3. Configure AWS Lambda

The last step is to log into the AWS Lamda console, upload the zip file and configure out Lambda with the following parameters:

  • Runtime: Java 11
  • Handler: jaotc.JaotCompilation::message

Furthermore, we need to create an environment variable with the name JAVA_TOOL_OPTIONS and set its value to:

-XX:+UnlockExperimentalVMOptions -XX:+PrintAOT -XX:AOTLibrary=./

This variable lets us pass parameters to the JVM.

The last step is to configure the input for our Lambda. The default is a JSON input, which cannot be passed to our function, therefore we need to set it to a String which contains an integer, e.g. “1”.

Finally, we can execute our Lambda function and should see in the log that our AOT compiled library was loaded:

57    1     loaded    ./  aot library

7. Conclusion

In this article, we saw how to AOT compile Java classes and modules. As this is still an experimental feature, the AOT compiler isn’t part of all distributions. Real examples are still rare to find, and it will be up to the Java community to find the best use cases for applying AOT.

All the code snippets in this article can be found in our GitHub repository.

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