fix: use default value for LoopController.continue_forever rather than initializing it in the constructor

src/core/src/main/java/org/apache/jmeter/control/LoopController.java

@@ -48,10 +48,6 @@ public class LoopController extends GenericController implements Serializable, I
 
     private boolean breakLoop;
 
-    public LoopController() {
-        set(getSchema().getContinueForever(), true);
-    }
-
     @Override
     public LoopControllerSchema getSchema() {
         return LoopControllerSchema.INSTANCE;

src/core/src/main/java/org/apache/jmeter/testelement/AbstractTestElement.java

@@ -482,10 +482,16 @@ public void setProperty(JMeterProperty property) {
         } else {
             writeLock();
             try {
-                propMap.put(property.getName(), property);
-                Map<String, JMeterProperty> propMapConcurrent = this.propMapConcurrent;
-                if (propMapConcurrent != null) {
-                    propMapConcurrent.put(property.getName(), property);
+                if (property instanceof StringProperty && property.getStringValue() == null) {
+                    // Avoid storing properties with null values since they will be skipped anyway when saving
+                    // the test plan
+                    removeProperty(property.getName());
+                } else {
+                    propMap.put(property.getName(), property);
+                    Map<String, JMeterProperty> propMapConcurrent = this.propMapConcurrent;
+                    if (propMapConcurrent != null) {
+                        propMapConcurrent.put(property.getName(), property);
+                    }
                 }
             } finally {
                 writeUnlock();

src/core/src/main/kotlin/org/apache/jmeter/control/LoopControllerSchema.kt

@@ -45,5 +45,5 @@ public abstract class LoopControllerSchema : GenericControllerSchema() {
      * However, it's not clear that a Thread Group could ever be repeated.
      */
     public val continueForever: BooleanPropertyDescriptor<LoopControllerSchema>
-        by boolean("LoopController.continue_forever")
+        by boolean("LoopController.continue_forever", default = true /* previous releases initialized value in constructor */)
 }

src/core/src/main/kotlin/org/apache/jmeter/dsl/DslPrinterTraverser.kt

@@ -173,7 +173,7 @@ public class DslPrinterTraverser(
             is DoubleProperty -> append(property.stringValue)
             is FloatProperty -> append(property.stringValue).append('f')
             is LongProperty -> append(property.stringValue).append('d')
-            is StringProperty -> appendLiteral(property.stringValue)
+            is StringProperty -> property.stringValue?.let { appendLiteral(it) } ?: run { append("null") }
 
             is TestElementProperty -> {
                 val element = property.element

src/dist-check/src/test/java/org/apache/jmeter/junit/JMeterTest.java

@@ -28,13 +28,15 @@
 import java.io.Serializable;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Modifier;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Properties;
 import java.util.stream.Collectors;
 
@@ -45,13 +47,16 @@
 
 import org.apache.commons.lang3.StringUtils;
 import org.apache.jmeter.config.gui.ObsoleteGui;
+import org.apache.jmeter.dsl.DslPrinterTraverser;
 import org.apache.jmeter.gui.JMeterGUIComponent;
 import org.apache.jmeter.gui.UnsharedComponent;
 import org.apache.jmeter.gui.tree.JMeterTreeNode;
 import org.apache.jmeter.save.SaveService;
 import org.apache.jmeter.testbeans.TestBean;
 import org.apache.jmeter.testbeans.gui.TestBeanGUI;
 import org.apache.jmeter.testelement.TestElement;
+import org.apache.jmeter.testelement.property.JMeterProperty;
+import org.apache.jmeter.testelement.property.PropertyIterator;
 import org.apache.jmeter.util.JMeterUtils;
 import org.apache.jorphan.reflect.ClassFinder;
 import org.apache.jorphan.util.JOrphanUtils;
@@ -270,6 +275,8 @@ private static Test suiteGUIComponents() throws Throwable {
                 System.out.println("o.a.j.junit.JMeterTest INFO: JMeterGUIComponent: skipping some tests " + item.getClass().getName());
             } else {
                 ts.addTest(new JMeterTest("GUIComponents2", item));
+                ts.addTest(new JMeterTest("saveLoadShouldKeepElementIntact", item));
+                ts.addTest(new JMeterTest("propertiesShouldNotBeInitializedToNullValues", item));
                 ts.addTest(new JMeterTest("runGUITitle", item));
             }
             suite.addTest(ts);
@@ -289,6 +296,8 @@ private static Test suiteBeanComponents() throws Throwable {
                 JMeterGUIComponent item = new TestBeanGUI(c);
                 TestSuite ts = new TestSuite(item.getClass().getName());
                 ts.addTest(new JMeterTest("GUIComponents2", item));
+                ts.addTest(new JMeterTest("saveLoadShouldKeepElementIntact", item));
+                ts.addTest(new JMeterTest("propertiesShouldNotBeInitializedToNullValues", item));
                 ts.addTest(new JMeterTest("runGUITitle", item));
                 suite.addTest(ts);
             } catch (IllegalArgumentException e) {
@@ -386,6 +395,75 @@ public void GUIComponents2() throws Exception {
         assertEquals("Modify Test: Failed on " + name, "hey, new name!:", el2.getName());
     }
 
+    public void propertiesShouldNotBeInitializedToNullValues() {
+        TestElement el = guiItem.createTestElement();
+        PropertyIterator it = el.propertyIterator();
+        while (it.hasNext()) {
+            JMeterProperty property = it.next();
+            if (property.getObjectValue() == null) {
+                fail(
+                        "Property " + property.getName() + " is initialized with NULL OBJECT value in " +
+                                " test element " + el + " created with " + guiItem + ".createTestElement() " +
+                                "Please refrain from that since null properties consume memory, and they will be " +
+                                "removed when saving and loading the plan anyway"
+                );
+            }
+            if (property.getStringValue() == null) {
+                fail(
+                        "Property " + property.getName() + " is initialized with NULL STRING value in " +
+                                " test element " + el + " created with " + guiItem + ".createTestElement() " +
+                                "Please refrain from that since null properties consume memory, and they will be " +
+                                "removed when saving and loading the plan anyway"
+                );
+            }
+        }
+    }
+
+    public void saveLoadShouldKeepElementIntact() throws IOException {
+        TestElement expected = guiItem.createTestElement();
+        ByteArrayOutputStream bos = new ByteArrayOutputStream();
+        SaveService.saveElement(expected, bos);
+        byte[] serializedBytes = bos.toByteArray();
+        TestElement actual = (TestElement) SaveService.loadElement(new ByteArrayInputStream(serializedBytes));
+        compareAllProperties(expected, actual, serializedBytes);
+    }
+
+    private static void compareAllProperties(TestElement expected, TestElement actual, byte[] serializedBytes) {
+        // JMeter restores "enabled" as StringProperty, see
+        // org.apache.jmeter.save.converters.ConversionHelp.restoreSpecialProperties
+        // So we normalize it back to BooleanProperty
+        JMeterProperty expEnabled = expected.getPropertyOrNull(expected.getSchema().getEnabled());
+        if (expEnabled != null && (expEnabled.getStringValue().equals("true") || expEnabled.getStringValue().equals("false"))) {
+            JMeterProperty actEnabled = actual.getPropertyOrNull(actual.getSchema().getEnabled());
+            if (actEnabled != null && actEnabled.getStringValue().equals(expEnabled.getStringValue())) {
+                actual.setProperty(expEnabled);
+            }
+        }
+
+        String expectedStr = new DslPrinterTraverser(DslPrinterTraverser.DetailLevel.ALL).append(expected).toString();
+        if (!Objects.equals(expected, actual)) {
+            boolean abc = Objects.equals(expected, actual);
+            assertEquals(
+                    "TestElement after 'save+load' should match the one created in GUI\n" +
+                            "JMX is " + new String(serializedBytes, StandardCharsets.UTF_8),
+                    expectedStr,
+                    new DslPrinterTraverser(DslPrinterTraverser.DetailLevel.ALL).append(actual).toString()
+            );
+            fail("TestElement after 'save+load' should match the one created in GUI. " +
+                    "DSL representation is the same, however TestElement#equals says the elements are different. " +
+                    "DSL is " + expectedStr + "\n" +
+                    "JMX is " + new String(serializedBytes, StandardCharsets.UTF_8));
+        }
+        assertEquals(
+                "TestElement.hashCode after 'save+load' should match the one created in GUI. " +
+                "DSL representation is the same, however TestElement#hashCode says the elements are different. " +
+                "DSL is " + expectedStr + "\n" +
+                "JMX is " + new String(serializedBytes, StandardCharsets.UTF_8),
+                expected.hashCode(),
+                actual.hashCode()
+        );
+    }
+
     /*
      * Test serializable elements - create the suite of tests
      */

src/dist-check/src/test/java/org/apache/jmeter/save/TestSaveService.java

@@ -117,7 +117,7 @@ private boolean loadAndSave(File testFile, String fileName, boolean checkSize, F
         final FileStats origStats = getFileStats(testFile);
         final FileStats savedStats = getFileStats(savedFile);
 
-        ByteArrayOutputStream out = new ByteArrayOutputStream(1000000);
+        ByteArrayOutputStream out = new ByteArrayOutputStream(Math.toIntExact(testFile.length()));
         try {
             HashTree tree = SaveService.loadTree(testFile);
             SaveService.saveTree(tree, out);

src/protocol/http/src/main/java/org/apache/jmeter/protocol/http/proxy/ProxyControl.java

@@ -45,6 +45,7 @@
 import java.util.Set;
 import java.util.prefs.Preferences;
 import java.util.regex.PatternSyntaxException;
+import java.util.stream.Collectors;
 
 import org.apache.commons.codec.binary.Base64;
 import org.apache.commons.codec.digest.DigestUtils;
@@ -398,11 +399,19 @@ public void setNotifyChildSamplerListenerOfFilteredSamplers(boolean b) {
     }
 
     public void setIncludeList(Collection<String> list) {
-        setProperty(new CollectionProperty(INCLUDE_LIST, new HashSet<>(list)));
+        if (list.size() >= 2) {
+            // Deduplicate if there is more than one element in the list
+            list = list.stream().distinct().collect(Collectors.toList());
+        }
+        setProperty(new CollectionProperty(INCLUDE_LIST, list));
     }
 
     public void setExcludeList(Collection<String> list) {
-        setProperty(new CollectionProperty(EXCLUDE_LIST, new HashSet<>(list)));
+        if (list.size() >= 2) {
+            // Deduplicate if there is more than one element in the list
+            list = list.stream().distinct().collect(Collectors.toList());
+        }
+        setProperty(new CollectionProperty(EXCLUDE_LIST, list));
     }
 
     public void setRegexMatch(boolean b) {

xdocs/changes.xml

@@ -109,6 +109,12 @@ Summary
 
 <ch_section>Bug fixes</ch_section>
 
+<h3>Thread Groups</h3>
+<ul>
+  <li><pr>6011</pr>Regression since 5.6: ThreadGroups are running endlessly in non-gui mode: use default value
+    for LoopController.continue_forever rather than initializing it in the constructor</li>
+</ul>
+
 <h3>HTTP Samplers and Test Script Recorder</h3>
 <ul>
 </ul>

xdocs/changes_history.xml

@@ -236,6 +236,8 @@ Apologies if we have omitted anyone else.
 
 <ch_section>Known problems and workarounds</ch_section>
 <ul>
+  <li><issue>6008</issue> ThreadGroups are running endlessly in non-gui mode (fixed in 5.6.1, see <pr>6011</pr>)</li>
+
   <li><pr>5987</pr>HTTP sampler sends filenames with percent-encoded UTF-8, however it is not aligned with the browsers. The workaround is to refrain non-ASCII filenames</li>
 
   <li><issue>6004</issue>Java Request sampler cannot be enabled again after disabling in UI (fixed in 5.6.1, <pr>6012</pr>)</li>