Document build with Ant

.github/workflows/java.yml

@@ -3,9 +3,8 @@ name: Java CI
 on: [push, pull_request]
 
 jobs:
-  build:
-    runs-on: ubuntu-latest
-
+  maven-ubuntu18:
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - run: sudo apt-get install protobuf-compiler
@@ -15,3 +14,40 @@ jobs:
           java-version: 1.8
       - name: Build with Maven
         run: mvn -B package --file pom.xml
+
+  maven-ubuntu20:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - run: sudo apt-get install protobuf-compiler
+      - name: Set up JDK 1.8
+        uses: actions/setup-java@v1
+        with:
+          java-version: 1.8
+      - name: Build with Maven
+        run: mvn -B package --file pom.xml
+
+  ant-ubuntu18:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - run: sudo apt-get install protobuf-compiler libprotobuf-java
+      - name: Set up JDK 1.8
+        uses: actions/setup-java@v1
+        with:
+          java-version: 1.8
+      - name: Build with Ant
+        run: ant
+
+  ant-ubuntu20:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - run: sudo apt-get install protobuf-compiler libprotobuf-java
+      - name: Set up JDK 1.8
+        uses: actions/setup-java@v1
+        with:
+          java-version: 1.8
+      - name: Build with Ant
+        run: ant
+

README.md

@@ -18,6 +18,8 @@ something more complete see [libosmium](https://osmcode.org/libosmium/).
 
 ## Java Version
 
+### Building with Maven
+
 We publish the Java library to [Maven Central](https://search.maven.org/):
 
 ```xml
@@ -34,8 +36,21 @@ To build the Java library run:
 mvn package
 ```
 
-For a Java usage example, see [`ReadFileTest`](https://github.com/openstreetmap/OSM-binary/blob/master/test.java/crosby/binary/ReadFileTest.java)
+For a Java usage example, see
+[`ReadFileTest`](https://github.com/openstreetmap/OSM-binary/blob/master/test.java/crosby/binary/ReadFileTest.java).
 
+### Building with Ant
+
+If you can not use Maven for some reason you can use the
+[Ant](https://ant.apache.org/) instead:
+
+```sh
+ant
+```
+
+This will build `osmpbf.jar` in the main directory.
+
+This build is also used for Debian packaging.
 
 ## C++ Version