import org.jenkinsci.plugins.workflow.steps.BodyExecutionCallback; //导入依赖的package包/类
@Override
public boolean start() throws Exception {
    EnvironmentExpander envEx = EnvironmentExpander.merge(getContext().get(EnvironmentExpander.class),
            EnvironmentExpander.constant(Collections.singletonMap("PATH+MODIFIER", step.getElement())));

    body = getContext().newBodyInvoker()
            .withContext(envEx)
            // Could use a dedicated BodyExecutionCallback here if we wished to print a message at the end ("Returning to ${cwd}"):
            .withCallback(BodyExecutionCallback.wrap(getContext()))
            .start();
    return false;

}