How to handle Java Callout errors

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What is a Java callout?

If you're new to Java callouts, we recommend that you start with How to create a Java callout.

Handling errors in a Java Callout

When you write a Java Callout, you may want to do custom error handling in your Java code. For example, you may wish to return custom error messages and headers and/or set flow variables with error information in the proxy flow on Edge.

Let's walk through a simple Java Callout example that illustrates basic custom error handling patterns. The sample returns a custom error message when an exception occurs. It also places the error stacktrace into a flow variable, which can be a handy debugging technique.

Download the project

To make things simple, you can download this project from the Apigee api-platform-samples repository on GitHub.

  1. Download or clone api-platform-samples to your system.
  2. In a terminal or code editor of your choice, go to the api-platform-samples/doc-samples/java-error project.

The sample Java code

The error handling patterns are straightforward. You can set flow variables in the current Edge flow context with the messageContext.setVariable() method. To return custom error information, construct an ExecutionResult instance and call methods on it to set the error response and headers.

package com.apigeesample;

import com.apigee.flow.execution.ExecutionContext;
import com.apigee.flow.execution.ExecutionResult;
import com.apigee.flow.execution.spi.Execution;
import com.apigee.flow.message.MessageContext;
import com.apigee.flow.execution.Action;

import org.apache.commons.lang.exception.ExceptionUtils;

public class JavaError implements Execution {
    public ExecutionResult execute(MessageContext messageContext, ExecutionContext executionContext) {

        try {

                String name = messageContext.getMessage().getHeader("username");

                if (name != null && name.length()>0) {
                        messageContext.getMessage().setContent("Hello, " + name + "!");

                } else {
                        throw new RuntimeException("Please specify a name parameter!");

                return ExecutionResult.SUCCESS;

        } catch (RuntimeException ex) {

            ExecutionResult executionResult = new ExecutionResult(false, Action.ABORT);

            //--Returns custom error message and header
            executionResult.addErrorResponseHeader("ExceptionClass", ex.getClass().getName());

            //--Set flow variables -- may be useful for debugging.
            messageContext.setVariable("JAVA_ERROR", ex.getMessage());
            messageContext.setVariable("JAVA_STACKTRACE", ExceptionUtils.getStackTrace(ex));
            return executionResult;

Compile your code with Maven

The project is set up so that you can compile with Maven. If you want to use javac, we'll include an example as well.

  1. Be sure that you have Maven installed:
    mvn -version
  2. Execute the script java-error/ This script installs the required JAR dependencies in your local Maven repo.
  3. cd to the java-error/callout directory.
  4. Execute Maven:
    mvn clean package
  5. If you wish, verify that the JAR file edge-custom-policy-java-error.jar was copied to java-error/apiproxy/resources/java. This is the required location for JAR files that you wish to deploy with a proxy.

Compile with javac

If you want to use javac to compile the code, you can do something similar to the following (from the java-error directory). The required JAR files are provided for you in the java-error/lib directory.

  1. cd to api-platform-samples/doc-samples/java-error.
  2. Be sure you have javac in your path.

    javac -version
  3. Execute the following javac command:

    javac -d . -classpath ./lib/expressions-1.0.0.jar:./lib/message-flow-1.0.0.jar:. callout/src/main/java/
  4. Copy the JAR file to the apiproxy/resources/java directory. This is the required location for JAR files that you wish to deploy with a proxy.

    cp com/apigeesample/JavaProperties.class apiproxy/resources/java

Deploy and call the proxy

A deploy script is provided in the ./java-error directory. But before you run it, you need to do a quick setup.

  1. cd to api-platform-samples/doc-samples/java-error
  2. If you haven't already done so, open the file ../../setup/ and edit it as indicated with your Apigee account information: your username (the email address associated with your account), your organization name, and the domain you use to make API management calls. For example, for Edge cloud, the domain is; however, your domain may be different if you're using Edge Private Cloud.
  3. Save the file.
  4. Execute the deploy script:
  5. If the deploy succeeds, execute the invoke script:

    The invoke script calls a cURL command that looks like this:

    curl  http://$org-$env.$api_domain/java-error

    Because the call does not include a "name" query parameter, the Java code throws a runtime error. The proxy returns this message and header:

  • Error message: Please specify a name parameter!
  • Header: ExceptionClass: java.lang.RuntimeException