Add option to skip Maven plugin execution (#5337)

* Add option to skip Maven plugin execution

The execution is skipped if either the codegen.skip property or the <skip>
configuration parameter is set. This is consistent with how many other Maven
plugins, such as maven-exec-plugin and maven-clean-plugin, handle this.

* Add documentation for Maven `skip` property

modules/swagger-codegen-maven-plugin/README.md

@@ -59,6 +59,7 @@ mvn clean compile
 - `generateModelDocumentation` - generate the model documentation (`true` by default. Only available if `generateModels` is `true`)
 - `generateSupportingFiles` - generate the supporting files (`true` by default)
 - `supportingFilesToGenerate` - A comma separated list of supporting files to generate.  All files is the default.
+- `skip` - skip code generation (`false` by default. Can also be set globally through the `codegen.skip` property)
 
 ### Custom Generator
 

modules/swagger-codegen-maven-plugin/src/main/java/io/swagger/codegen/plugin/CodeGenMojo.java

@@ -228,7 +228,11 @@ public class CodeGenMojo extends AbstractMojo {
     @Parameter(name = "generateApiDocumentation", required = false)
     private Boolean generateApiDocumentation = true;
 
-
+    /**
+     * Skip the execution.
+     */
+    @Parameter(name = "skip", property = "codegen.skip", required = false, defaultValue = "false")
+    private Boolean skip;
 
     /**
      * Add the output directory to the project as a source root, so that the
@@ -252,6 +256,14 @@ public class CodeGenMojo extends AbstractMojo {
     @Override
     public void execute() throws MojoExecutionException {
 
+        if(skip) {
+            getLog().info("Code generation is skipped.");
+            // Even when no new sources are generated, the existing ones should
+            // still be compiled if needed.
+            addCompileSourceRootIfConfigured();
+            return;
+        }
+
         //attempt to read from config file
         CodegenConfigurator configurator = CodegenConfigurator.fromFile(configurationFile);
 
@@ -427,7 +439,11 @@ public class CodeGenMojo extends AbstractMojo {
             throw new MojoExecutionException("Code generation failed. See above for the full exception.");
         }
 
-        if (addCompileSourceRoot) {
+        addCompileSourceRootIfConfigured();
+    }
+
+    private void addCompileSourceRootIfConfigured() {
+        if(addCompileSourceRoot) {
             final Object sourceFolderObject = configOptions == null ? null : configOptions.get(CodegenConstants.SOURCE_FOLDER);
             final String sourceFolder =  sourceFolderObject == null ? "src/main/java" : sourceFolderObject.toString();