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+# Building Babylon
+[Back to Index ../](../index.md)
+
+OpenJDK Babylon can be found here [https://github.com/openjdk/babylon](https://github.com/openjdk/babylon)
+
+If you follow the steps below to build Babylon, you should not have to
+change any of the `maven` or `cmake` build files for hat.
+
+## Some useful vars
+You will need an existing version of JDK to build Babylon and [jtreg](https://github.com/openjdk/jtreg).
+
+The following build process assumes you have an env var called
+`BOOT_JDK` set to an existing JDK ([JDK 26+](https://jdk.java.net/26/)).
+
+For example assuming you have just installed java 26 on your machine into `${HOME}/java/jdk-26.jdk`
+```bash
+export BOOT_JDK=${HOME}/java/jdk-26.jdk/Contents/Home/
+```
+
+We will also assume that you have a `GITHUB` var pointing to location where you pull
+repo's from GITHUB.
+
+For example I always pull all my Babylon/HAT related repos in `${HOME}/github` so..
+```bash
+export GITHUB=${HOME}/github
+```
+
+### Clone jtreg from GitHub ([https://github.com/openjdk/jtreg](https://github.com/openjdk/jtreg))
+```bash
+cd ${GITHUB}
+git clone https://github.com/openjdk/jtreg
+```
+### Build jtreg
+```bash
+export JTREG_ROOT=${GITHUB}/jtreg
+cd ${JTREG_ROOT}
+bash make/build.sh --jdk ${BOOT_JDK}
+```
+We will need to point a var called JTREG at the resulting image
+
+```bash
+export JTREG_HOME=${JTREG_ROOT}/build/images/jtreg
+```
+
+### Clone Babylon from GitHub ([https://github.com/opendjk/babylon.git](https://github.com/opendjk/babylon.git))
+```bash
+cd ${GITHUB}
+git clone https://github.com/opendjk/babylon.git
+```
+### Configure Babylon
+```bash
+cd ${GITHUB}/babylon
+bash configure  --with-boot-jdk=${BOOT_JDK} --with-jtreg=${JTREG_HOME}
+```
+If you have never built JDK before you may find that the `configure` step will suggest packages to install.
+I usually just keep running `bash configure` and take the suggestions (missing packages) I you get a successful Babylon build.
+
+### Build Babylon
+
+```bash
+make clean
+make images
+#Coffee time (between 2 mins and 10 mins usually depends on your machine)
+```
+You now should have:
+
+```bash
+github
+├── babylon
+│   ├── build
+│   │   └── XXXX-server-release
+│   │       ├── Makefile
+│   │       ├── jdk
+│   │       ├── images
+│   │       ├── hotspot
+│   │       └── ...
+│   ├── hat
+│   │   ├── ...
+```
+
+Where XXXX will depend on your architecture AARCH64 or X64_86
+
+### Run can now JTREG Tests
+If we included `jtreg` above we can run the `babylon` code reflection tests using
+```bash
+cd ${GITHUB}/babylon
+make test TEST=jdk_lang_reflect_code
+```
+This works because babylon added added:
+```txt
+8<-
+jdk_lang_reflect_code = \
+   java/lang/reflect/code
+->8
+```
+To the file `${GITHUB}/babylon/test/jdk/TEST.groups`
+
+For the curious...the tests themselves can be found in this directory
+```bash
+tree {GITHUB}/babylon}/test/jdk/java/lang/reflect/code
+```
+[Next Building HAT](hat.md)