Perform retries (on UnknownHostExceptions)

If an UnknownHostException occurs (#270), we retry (configurable)
times, as this is sometimes due to DNS (caching) errors
with Java (or the underlying OS).

Fix typos and wording (#343).

htmlSanityCheck-core/src/main/java/org/aim42/htmlsanitycheck/Configuration.java

@@ -45,6 +45,8 @@ public class Configuration {
     @Getter(AccessLevel.NONE)
     @Builder.Default
     Boolean ignoreIPAddresses = false;
+    @Builder.Default
+    Integer retries = 0;
     /*
      * Explanation for configuring http status codes:
      * The standard http status codes are defined in class @link NetUtil and can

htmlSanityCheck-core/src/main/java/org/aim42/htmlsanitycheck/check/BrokenHttpLinksChecker.java

@@ -17,7 +17,7 @@
 
 
 /**
- * Check html anchor href attributes
+ * Check HTML anchor href attributes
  *
  * @see <a href="https://www.w3schools.com/tags/att_a_href.asp">https://www.w3schools.com/tags/att_a_href.asp</a>
  */
@@ -32,15 +32,13 @@ class BrokenHttpLinksChecker extends Checker {
     private final Set<Integer> successCodes;
     private final Set<Integer> warningCodes;
     private final Set<Integer> errorCodes;
-    // all href attributes with http(s) protocol,
-    // including potential duplicates
-    // need that to calculate "nrOfOccurrences"
-    // the pure http/https-hrefs a set, duplicates are removed here
+    // all href attributes with http(s) protocol, including potential duplicates, need
+    // to calculate "nrOfOccurrences" the pure http/https-hrefs a set, duplicates are removed here
     private Set<String> hrefSet;
 
 
-    BrokenHttpLinksChecker(Configuration pConfig) {
-        super(pConfig);
+    BrokenHttpLinksChecker(Configuration configuration) {
+        super(configuration);
 
         errorCodes = getMyConfig().getHttpErrorCodes();
         warningCodes = getMyConfig().getHttpWarningCodes();
@@ -59,7 +57,7 @@ protected void initCheckingResultsDescription() {
     protected SingleCheckResults check(final HtmlPage pageToCheck) {
         log.trace("Checking '{}'", pageToCheck.getFile());
 
-        //get set of all a-tags "<a href=..." in html file,
+        // get a set of all a-tags "<a href=..." in HTML file,
         // restricted to http(s) links
 
         hrefSet = pageToCheck.getAllHttpHrefStringsAsSet();
@@ -82,11 +80,11 @@ private void addWarningIfNoInternetConnection() {
     }
 
     /**
-     * check all http(s) links
+     * Check all http(s) links
      * TODO: use GPARS to check several links in parallel, as sequential checking might take too long
      **/
     private void checkAllHttpLinks() {
-        // for all hrefSet check if the corresponding link is valid
+        // Check if the corresponding link is valid for all hrefSet
         hrefSet.forEach(this::doubleCheckSingleHttpLink);
     }
 
@@ -98,8 +96,6 @@ private void checkAllHttpLinks() {
      * we try again with a GET, to get the "finalResponseCode" -
      * which we then categorize as success, error or warning
      */
-
-
     protected void doubleCheckSingleHttpLink(String href) {
         // bookkeeping:
         getCheckingResults().incNrOfChecks();
@@ -108,14 +104,14 @@ protected void doubleCheckSingleHttpLink(String href) {
             URL url = new URL(href);
             checkIfLocalhostURL(url, href);
             checkIfIPAddress(url, href);
-            checkHttpLinkWithRetry(url, href);
+            checkHttpLinkWithRetry(url, href, getMyConfig().getRetries());
         } catch (MalformedURLException exception) {
             Finding malformedURLFinding = new Finding("malformed URL exception with href=" + href);
             getCheckingResults().addFinding(malformedURLFinding);
         }
     }
 
-    private void checkHttpLinkWithRetry(URL url, String href) {
+    private void checkHttpLinkWithRetry(URL url, String href, int retries) {
         String problem;
         try {
             HttpURLConnection firstConnection = getNewURLConnection(url);
@@ -124,7 +120,7 @@ private void checkHttpLinkWithRetry(URL url, String href) {
             firstConnection.connect();
             int responseCode = firstConnection.getResponseCode();
 
-            // issue 218 and 219: some web servers respond with 403 or 405
+            // Issue 218 and 219: some web servers respond with 403 or 405
             // when given HEAD requests. Therefore, try to GET
             if (successCodes.contains(responseCode)) {
                 return;
@@ -169,8 +165,13 @@ else if (Web.HTTP_REDIRECT_CODES.contains(responseCode)) {
             firstConnection.disconnect();
 
         } catch (UnknownHostException exception) {
-            Finding unknownHostFinding = new Finding("Unknown host with href=" + href, exception);
-            getCheckingResults().addFinding(unknownHostFinding);
+            if (retries > 0) {
+                log.warn("Unknown host exception with href={}, retrying another {} times", href, retries);
+                checkHttpLinkWithRetry(url, href, retries - 1);
+            } else {
+                Finding unknownHostFinding = new Finding("Unknown host with href=" + href, exception);
+                getCheckingResults().addFinding(unknownHostFinding);
+            }
         } catch (IOException exception) {
             Finding someException = new Finding("exception " + exception + " with href=" + href, exception);
             getCheckingResults().addFinding(someException);
@@ -212,12 +213,12 @@ private void checkIfIPAddress(URL url, String href) {
         }
     }
 
-    // if configured ,localhost-URLs yield warnings!
+    // if configured, localhost-URLs yield warnings!
     private void checkIfLocalhostURL(URL url, String href) {
         if (!getMyConfig().isIgnoreLocalhost()) {
             String host = url.getHost();
             if (("localhost".equals(host)) || host.startsWith("127.0.0")) {
-                Finding localhostWarning = new Finding("Warning: localhost urls indicates suspicious environment dependency: href=" + href);
+                Finding localhostWarning = new Finding("Warning: localhost urls indicate suspicious environment dependency: href=" + href);
                 getCheckingResults().addFinding(localhostWarning);
             }
         }

htmlSanityCheck-core/src/test/groovy/org/aim42/htmlsanitycheck/check/BrokenHttpLinksCheckerSpec.groovy

@@ -20,7 +20,6 @@ class BrokenHttpLinksCheckerSpec extends Specification {
     HtmlPage htmlPage
     SingleCheckResults collector
 
-    private Configuration myConfig
     static private int port
 
     @Shared
@@ -41,8 +40,10 @@ class BrokenHttpLinksCheckerSpec extends Specification {
     /* executed before every single spec */
 
     def setup() {
-        myConfig = new Configuration()
-        brokenHttpLinksChecker = new BrokenHttpLinksChecker(myConfig)
+        Configuration configuration = Configuration.builder()
+                .retries(3)
+                .build()
+        brokenHttpLinksChecker = new BrokenHttpLinksChecker(configuration)
 
         collector = new SingleCheckResults()
     }

htmlSanityCheck-gradle-plugin/README.adoc

@@ -28,8 +28,8 @@ plugins {
     id 'org.aim42.{project}' version '{hsc-version}' // <1>
 }
 ----
-<1> Checkout <<box:current-version,current version>>
 
+<1> Checkout <<box:current-version,current version>>
 
 === Legacy Installation
 
@@ -52,6 +52,7 @@ buildscript {
 
 apply plugin: 'org.aim42.{project}'
 ----
+
 <1> In case you would like to use a development version (or even branch), check out <<sec:development-versions,development versions>>.
 <2> Beginning with version `2.x` all releases will be published to https://central.sonatype.com/artifact/org.aim42.htmlSanityCheck/org.aim42.htmlSanityCheck.gradle.plugin[Maven Central].
 <3> The https://plugins.gradle.org[Gradle Plugin Portal] contains https://plugins.gradle.org/plugin/org.aim42.htmlSanityCheck[most versions] or will redirect downloads of newer versions to Maven Central.
@@ -61,8 +62,8 @@ apply plugin: 'org.aim42.{project}'
 [IMPORTANT]
 .Latest (development) versions
 ====
-* The latest https://github.com/aim42/htmlSanityCheck/releases[release version] is defined in https://github.com/aim42/htmlSanityCheck/blob/main/gradle.properties[`gradle.properties` on `main` branch].
-* The current (development) version is defined in {gradleProperties}
+* https://github.com/aim42/htmlSanityCheck/blob/main/gradle.properties[`gradle.properties` on `main` branch] defines the latest https://github.com/aim42/htmlSanityCheck/releases[release version].
+* {gradleProperties} defines the current (development) version.
 +
 [source]
 .Development version
@@ -79,7 +80,7 @@ The plugin adds a new task named `htmlSanityCheck`.
 This task exposes a few properties as part of its configuration:
 
 [horizontal]
-`sourceDir` (mandatory):: Directory where the HTML files are located.
+`sourceDir` (mandatory):: Directory containing the HTML files to check.
 +
 Type: Directory.
 
@@ -95,14 +96,14 @@ Type: Directory.
 +
 Default: `+{buildDir}+/reports/htmlSanityCheck/`
 
-`junitResultsDir` (optional):: Directory where the results are written to in JUnit XML format.
-JUnit XML can be read by many tools, including CI environments.
+`junitResultsDir` (optional):: Directory where HSC writes the results in JUnit XML format.
+Many tools support https://github.com/testmoapp/junitxml[JUnit XML] format, including CI environments.
 +
 Type: Directory.
 +
 Default: `+{buildDir}+/test-results/htmlchecks/`
 
-`failOnErrors` (optional):: Fail the build if any error was found in the checked pages.
+`failOnErrors` (optional):: Fail the build if any checked of the pages contains an error.
 +
 Type: Boolean.
 +
@@ -126,6 +127,15 @@ Type: Boolean.
 +
 Default: `false`.
 
+`retries` (optional):: Retry on failed requests (e.g., network issues).
++
+*Note*: Currently only used for `UnknownHostException`s (cf.
+{project-issues}/270[#270]).
++
+Type: Integer.
++
+Default: `0`.
+
 `checkerClasses` (optional):: The set of checker classes to be executed.
 +
 Type: List.
@@ -155,12 +165,11 @@ include::../htmlSanityCheck-core/src/main/java/org/aim42/htmlsanitycheck/tools/W
 [NOTE]
 .HTTP Redirects
 ====
-Note that HTTP redirects are treated as a warning to make the user aware of the correct or new location (cf. {project-issues}/244[Issue 244]).
+Note that HSC treats HTTP redirects as a warning to make the user aware of the correct or new location (cf.
+{project-issues}/244[Issue 244]).
 Some HSC reports often contain the respective location.
 ====
 
-
-
 `httpErrorCodes` (optional):: Additional HTTP response codes treated as error.
 +
 Type: List.
@@ -190,12 +199,13 @@ The lists shown above are the default HTTP response codes handled by HSC.
 The mentioned configurations effectively move the configured codes around, i.e., if you add `308` to `httpErrorCodes` it is automatically removed from its default list (`httpWarningCodes`).
 ****
 
-
 [[sec:examples]]
 == Examples
 
 === Small Example
+
 `build.gradle`
+
 [source,groovy]
 ----
 apply plugin: 'org.aim42.htmlSanityCheck'
@@ -269,8 +279,6 @@ asciidoctor {
         from( srcImagesPath )
         into targetImagesPath
     }
-
-
 }
 
 apply plugin: 'org.aim42.htmlSanityCheck'
@@ -320,7 +328,7 @@ htmlSanityCheck {
 
 == Compatibility
 
-The Plugin has been tested with the following Gradle versions:
+We test the Plugin with the following Gradle versions:
 
 [source,groovy]
 .Tested Gradle versions
@@ -330,7 +338,7 @@ include::src/test/groovy/org/aim42/htmlsanitycheck/gradle/HtmlSanityCheckTaskFun
 
 [NOTE]
 ====
-The full range of Gradle versions is only tested in CI (GitHub action).
+We test the full range of Gradle versions only in CI (GitHub action) as it is time-consuming.
 The local test only tests the latest Gradle version:
 
 [source,groovy]

htmlSanityCheck-gradle-plugin/src/main/groovy/org/aim42/htmlsanitycheck/gradle/HtmlSanityCheckTask.groovy

@@ -83,6 +83,9 @@ class HtmlSanityCheckTask extends DefaultTask {
     @Input
     List<Class<? extends Checker>> checkerClasses = AllCheckers.CHECKER_CLASSES
 
+    @Input
+    Integer retries = 0
+
     // private stuff
     // **************************************************************************
     private Configuration myConfig
@@ -187,6 +190,7 @@ See ${checkingResultsDir} for a detailed report."""
                 .ignoreIPAddresses(ignoreIPAddresses)
 
                 .checksToExecute(checkerClasses)
+                .retries(retries)
                 .build()
 
         // in case we have configured specific interpretations of http status codes

integration-test/gradle-plugin/build.gradle

@@ -65,13 +65,13 @@ task buildReadmeGradlePlugin(type: org.asciidoctor.gradle.jvm.AsciidoctorTask) {
     )
 }
 buildReadmeGradlePlugin.dependsOn(buildReadmeRoot)
-//buildReadmeGradlePlugin.dependsOn(copyReadmeResources)
 
 htmlSanityCheck {
     sourceDir file("build/docs")
 
     checkingResultsDir = file("build/reports")
     failOnErrors = true
+    retries = 3
 
     logger.quiet "HSC version: ${htmlSanityCheckVersion}"
     logger.quiet "HSC sourceDir: ${sourceDir.absolutePath}"