Adds detailed instructions on how to execute ExampleEchoPipeline.java (…

…apache#33108)

* Adds detailed instructions on how to execute ExampleEchoPipeline.java

* spacing in exampleechopipeline

* revert header

* spacing

* spacing

* spacing

* license

* spotless

examples/java/src/main/java/org/apache/beam/examples/subprocess/ExampleEchoPipeline.java

@@ -33,16 +33,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-/**
- * In this example batch pipeline we will invoke a simple Echo C++ library within a DoFn The sample
- * makes use of a ExternalLibraryDoFn class which abstracts the setup and processing of the
- * executable, logs and results. For this example we are using commands passed to the library based
- * on ordinal position but for a production system you should use a mechanism like ProtoBuffers with
- * Base64 encoding to pass the parameters to the library To test this example you will need to build
- * the files Echo.cc and EchoAgain.cc in a linux env matching the runner that you are using (using
- * g++ with static option). Once built copy them to the SourcePath defined in {@link
- * SubProcessPipelineOptions}
- */
+/** Please see the Readme.MD file for instructions to execute this pipeline. */
 public class ExampleEchoPipeline {
 
   public static void main(String[] args) throws Exception {

examples/java/src/main/java/org/apache/beam/examples/subprocess/Readme.MD

@@ -0,0 +1,86 @@
+<!--
+    Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+-->
+# Apache Beam Subprocess Example
+
+This example demonstrates how to execute external C++ binaries as subprocesses within an Apache Beam pipeline using the `SubProcessKernel`.
+
+## Prerequisites
+
+*   **Google Cloud Project:** A Google Cloud project with billing enabled.
+*   **Dataflow API:** Enable the Dataflow API for your project.
+*   **C++ compiler:** You'll need a C++ compiler (like g++) to compile the C++ binaries.
+
+## Steps
+
+1. **Create a [Maven Example project](https://beam.apache.org/get-started/quickstart-java/) that builds against the latest Beam release:**
+
+    ```bash
+    mvn archetype:generate \
+    -DarchetypeGroupId=org.apache.beam \
+    -DarchetypeArtifactId=beam-sdks-java-maven-archetypes-examples \
+    -DarchetypeVersion=2.60.0 \
+    -DgroupId=org.example \
+    -DartifactId=word-count-beam \
+    -Dversion="0.1" \
+    -Dpackage=org.apache.beam.examples \
+    -DinteractiveMode=false
+    ```
+
+2. **Build the project:**
+
+    *   Navigate to the root of the repository (`word-count-beam/`):
+
+        ```bash
+        cd word-count-beam/
+        ```
+
+    *   Build the project using Maven:
+
+        ```bash
+        mvn clean install
+        ```
+
+3. **Run the pipeline on Dataflow:**
+
+    ```bash
+    mvn compile exec:java \
+      -Dexec.mainClass=org.apache.beam.examples.subprocess.ExampleEchoPipeline \
+      -Dexec.args="--sourcePath=/absolute/path/to/your/subprocess/directory \
+                   --workerPath=/absolute/path/to/your/subprocess/directory \
+                   --concurrency=5 \
+                   --filesToStage=/absolute/path/to/your/subprocess/directory/echo,/absolute/path/to/your/subprocess/directory/echoagain \
+                   --runner=DataflowRunner \
+                   --project=your-project-id \
+                   --region=your-gcp-region \
+                   --tempLocation=gs://your-gcs-bucket/temp"
+    ```
+
+    *   Replace the placeholders with your actual paths, project ID, region, and Cloud Storage bucket.
+
+## Important notes
+
+* **Dependencies:** Ensure your `pom.xml` includes the Dataflow runner dependency (`beam-runners-google-cloud-dataflow-java`).
+* **Authentication:** Authenticate your environment to your Google Cloud project.
+* **DirectRunner:** On `DirectRunner`, you will see the error ` Process succeded but no result file was found`, showing that the Process is successful.
+
+## Code overview
+
+*   **`ExampleEchoPipeline.java`:** This Java file defines the Beam pipeline that executes the `Echo` and `EchoAgain` binaries as subprocesses.
+*   **`Echo.cc` and `Echoagain.cc`:** These C++ files contain the code for the external binaries. These won't be visible when running the example with the created example project. You will need to compile these (using `g++ Echo.cc -o Echo` and `g++ EchoAgain.cc -o EchoAgain`), and then provide their path via the `sourcePath` and `workerPath` flags as listed above.
+*   **`SubProcessKernel.java`:** This class in the Beam Java SDK handles the execution of external binaries and captures their output.