Issue checkstyle#6232: Load resources relative to resources root

src/main/java/com/puppycrawl/tools/checkstyle/utils/CommonUtil.java

@@ -504,8 +504,9 @@ public static URI getUriByFilename(String filename) throws CheckstyleException {
             else {
                 // check to see if the file is in the classpath
                 try {
-                    final URL configUrl = CommonUtil.class
-                            .getResource(filename);
+                    final URL configUrl = filename.startsWith("/") ?
+                            CommonUtil.class.getResource(filename) :
+                            ClassLoader.getSystemResource(filename);
                     if (configUrl == null) {
                         throw new CheckstyleException(UNABLE_TO_FIND_EXCEPTION_PREFIX + filename);
                     }

src/test/java/com/puppycrawl/tools/checkstyle/internal/powermock/CommonUtilPowerTest.java

@@ -24,6 +24,7 @@
 import static org.junit.Assert.fail;
 import static org.powermock.api.mockito.PowerMockito.mock;
 import static org.powermock.api.mockito.PowerMockito.mockStatic;
+import static org.powermock.api.mockito.PowerMockito.spy;
 import static org.powermock.api.mockito.PowerMockito.when;
 
 import java.net.URISyntaxException;
@@ -32,6 +33,7 @@
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.mockito.Mockito;
+import org.powermock.api.mockito.PowerMockito;
 import org.powermock.core.classloader.annotations.PrepareForTest;
 import org.powermock.modules.junit4.PowerMockRunner;
 
@@ -49,7 +51,7 @@ public void testLoadSuppressionsUriSyntaxException() throws Exception {
 
         when(configUrl.toURI()).thenThrow(URISyntaxException.class);
         mockStatic(CommonUtil.class, Mockito.CALLS_REAL_METHODS);
-        final String fileName = "suppressions_none.xml";
+        final String fileName = "/suppressions_none.xml";
         when(CommonUtil.class.getResource(fileName)).thenReturn(configUrl);
 
         try {

src/test/java/com/puppycrawl/tools/checkstyle/utils/CommonUtilTest.java

@@ -20,19 +20,29 @@
 package com.puppycrawl.tools.checkstyle.utils;
 
 import static com.puppycrawl.tools.checkstyle.internal.utils.TestUtil.isUtilsClassHasPrivateConstructor;
+import static org.hamcrest.CoreMatchers.containsString;
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.CoreMatchers.not;
+import static org.hamcrest.CoreMatchers.nullValue;
+import static org.hamcrest.CoreMatchers.startsWith;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
+import java.io.ByteArrayOutputStream;
 import java.io.Closeable;
 import java.io.File;
 import java.io.IOException;
+import java.io.InputStream;
 import java.lang.reflect.Constructor;
+import java.net.URI;
 import java.util.Dictionary;
 import java.util.regex.Pattern;
 
+import org.apache.commons.io.IOUtils;
 import org.junit.Test;
 
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
@@ -42,6 +52,8 @@ public class CommonUtilTest {
 
     /** After appending to path produces equivalent, but denormalized path. */
     private static final String PATH_DENORMALIZER = "/levelDown/.././";
+    private static final String PACKAGE_DIR_NAME = CommonUtilTest.class.getPackage().getName().replaceAll("\\.", "/");
+    private static final String SIMPLE_CLASS_NAME = CommonUtilTest.class.getSimpleName();
 
     @Test
     public void testIsProperUtilsClass() throws ReflectiveOperationException {
@@ -440,6 +452,30 @@ public void testIsIntNull() {
             CommonUtil.isInt(null));
     }
 
+    @Test
+    public void testGetUriByFilenameFindsAbsoluteResourceOnClasspath() throws Throwable {
+        URI uri = CommonUtil.getUriByFilename("/" + PACKAGE_DIR_NAME + "/commonutil/Input" + SIMPLE_CLASS_NAME + "_empty_checks.xml");
+        assertThat(uri, is(not(nullValue())));
+    }
+
+    @Test
+    public void testGetUriByFilenameFindsRelativeResourceOnClasspath() throws Throwable {
+        URI uri = CommonUtil.getUriByFilename(PACKAGE_DIR_NAME + "/commonutil/Input" + SIMPLE_CLASS_NAME + "_empty_checks.xml");
+        assertThat(uri, is(not(nullValue())));
+    }
+
+    @Test
+    public void testGetUriByFilenameFindsResourceRelativeToRootClasspath() throws Throwable {
+        URI uri = CommonUtil.getUriByFilename("commonutil/Input" + SIMPLE_CLASS_NAME + "_resource.txt");
+        assertThat(uri, is(not(nullValue())));
+        assertThat(uri.toString(), not(containsString(CommonUtil.class.getPackage().getName().replaceAll("\\.", "/"))));
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        try (InputStream in = uri.toURL().openStream()) {
+            IOUtils.copy(in, out);
+        }
+        assertThat(new String(out.toByteArray()), startsWith("good"));
+    }
+
     private static class TestCloseable implements Closeable {
 
         private boolean closed;

src/test/resources/com/puppycrawl/tools/checkstyle/utils/commonutil/InputCommonUtilTest_empty_checks.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+        "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
+        "http://checkstyle.sourceforge.net/dtds/configuration_1_3.dtd">
+<module name="Checker"/>

src/test/resources/com/puppycrawl/tools/checkstyle/utils/commonutil/InputCommonUtilTest_resource.txt

@@ -0,0 +1 @@
+bad

src/test/resources/commonutil/InputCommonUtilTest_resource.txt

@@ -0,0 +1 @@
+good