Create CI build

.github/workflows/CI.yml

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+name: Java CI with Gradle
+
+on:
+  push:
+    branches: [ "main" ]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        java-version: [ '17', '18', '19', '20', '21' ]
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout project
+      uses: actions/checkout@c85c95e3d7251135ab7dc9ce3241c5835cc595a9 # v3.5.3
+
+    # Gradle java toolchain cannot find early release images, need to preload until GA is available
+    - name: Preload JDK 21
+      if: ${{ matrix.java-version == '21' }}
+      uses: actions/setup-java@5ffc13f4174014e2d4d4572b3d74c3fa61aeb2c2 # v3.11.0
+      with:
+        java-version: '21-ea'
+        distribution: 'temurin'
+
+    - name: Set up JDK 11
+      uses: actions/setup-java@5ffc13f4174014e2d4d4572b3d74c3fa61aeb2c2 # v3.11.0
+      with:
+        java-version: '11'
+        distribution: 'temurin'
+        cache: gradle    
+
+    - name: Build application
+      run: |
+        ./gradlew io.openliberty.java.internal_fat_${{ matrix.java-version }}:build
+
+    - name: Verify application
+      run: |
+        ./.github/workflows/scripts/verifyArtifact.sh ${{ matrix.java-version }}
+    
+    - name: Upload application
+      uses: actions/upload-artifact@013d2b89baa2f354c5ffec54c68bec4ab39a2534 #v3.1.2
+      with:
+        name: Applications
+        path: io.openliberty.java.internal_fat_${{ matrix.java-version }}/build/libs/io.openliberty.java.internal_fat_${{ matrix.java-version }}.war
+        if-no-files-found: error

.github/workflows/scripts/ClassVersionChecker.java

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+import java.io.*;
+import java.util.*;
+
+public class ClassVersionChecker {
+
+    private static final HashMap<String, Integer> majorCodeMap = new HashMap<>();
+
+    public static void main(String[] args) throws IOException {
+        if (args.length != 2) {
+            throw new RuntimeException("Expected exactly 2 arguments");
+        }
+
+        //Adapted from https://docs.oracle.com/javase/specs/jvms/se20/html/jvms-4.html#jvms-4.1
+        majorCodeMap.put("1.0.2", 45);
+        majorCodeMap.put("1.1", 45);
+        majorCodeMap.put("1.2", 46);
+        majorCodeMap.put("1.3", 47);
+        majorCodeMap.put("1.4", 48);
+        majorCodeMap.put("5.0", 49);
+        majorCodeMap.put("6", 50);
+        majorCodeMap.put("7", 51);
+        majorCodeMap.put("8", 52);
+        majorCodeMap.put("9", 53);
+        majorCodeMap.put("10", 54);
+        majorCodeMap.put("11", 55);
+        majorCodeMap.put("12", 56);
+        majorCodeMap.put("13", 57);
+        majorCodeMap.put("14", 58);
+        majorCodeMap.put("15", 59);
+        majorCodeMap.put("16", 60);
+        majorCodeMap.put("17", 61);
+        majorCodeMap.put("18", 62);
+        majorCodeMap.put("19", 63);
+        majorCodeMap.put("20", 64);
+        majorCodeMap.put("21", 65);
+
+        String filename = args[0];
+        int expected = majorCodeMap.get(args[1]);
+
+        checkClassVersion(filename, expected);
+    }
+
+    private static void checkClassVersion(String filename, int expected) throws IOException {
+        try ( DataInputStream in = new DataInputStream(new FileInputStream(filename)) ) {
+            int magic = in.readInt();
+
+            if(magic != 0xcafebabe) {
+                System.out.println(filename + " is not a valid class!");
+            }
+
+            int minor = in.readUnsignedShort();
+            int major = in.readUnsignedShort();
+
+            System.out.println("Major version: " + major);
+            System.out.println("Minor version: " + minor);
+
+            if(expected != major) {
+                System.out.println("Expected major version of " + expected + " but got " + major);
+                System.exit(1); //Failing execution
+            }
+        }
+    }
+}

.github/workflows/scripts/verifyArtifact.sh

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+#!/bin/bash
+
+if [ $# -ne 1 ]; then
+    echo "Exactly one argument is required"
+    exit 0
+fi
+
+JAVA_VERSION=$1
+SUB_PROJECT=$PWD/io.openliberty.java.internal_fat_$JAVA_VERSION
+TEST_CLASS=$SUB_PROJECT/build/classes/java/main/io/openliberty/java/internal/TestApp.class
+
+
+echo "Checking class"
+SCRIPTS=$PWD/.github/workflows/scripts/
+pushd $SCRIPTS &> /dev/null
+    java ClassVersionChecker.java $TEST_CLASS $JAVA_VERSION
+popd &> /dev/null
+echo "--------------------"