Restore old serialized FileNode instances correctly

pom.xml

@@ -45,7 +45,7 @@
 
   <properties>
     <scmTag>HEAD</scmTag>
-    <revision>0.23.0</revision>
+    <revision>0.22.1</revision>
     <changelist>-SNAPSHOT</changelist>
 
     <module.name>edu.hm.hafner.coverage</module.name>

src/main/java/edu/hm/hafner/coverage/FileNode.java

@@ -29,7 +29,7 @@
  */
 @SuppressWarnings("PMD.GodClass")
 public final class FileNode extends Node {
-    private static final long serialVersionUID = -3795695377267542624L;
+    private static final long serialVersionUID = -3795695377267542624L; // Set to 1 when release 1.0.0 is ready
 
     private final NavigableMap<Integer, Integer> coveredPerLine = new TreeMap<>();
     private final NavigableMap<Integer, Integer> missedPerLine = new TreeMap<>();
@@ -40,7 +40,7 @@ public final class FileNode extends Node {
     private final NavigableMap<Integer, Integer> indirectCoverageChanges = new TreeMap<>();
     private final NavigableMap<Metric, Fraction> coverageDelta = new TreeMap<>();
 
-    private TreeString relativePath;
+    private TreeString relativePath; // @since 0.22.0
 
     /**
      * Creates a new {@link FileNode} with the given name.
@@ -68,6 +68,18 @@ public FileNode(final String name, final String relativePath) {
         this(name, TreeString.valueOf(relativePath));
     }
 
+    /**
+     * Called after de-serialization to retain backward compatibility.
+     *
+     * @return this
+     */
+    protected Object readResolve() {
+        if (relativePath == null) {
+            relativePath = TreeString.valueOf(StringUtils.EMPTY);
+        }
+        return this;
+    }
+
     @Override
     public FileNode copy() {
         var file = new FileNode(getName(), relativePath);

src/test/java/edu/hm/hafner/coverage/AbstractNodeTest.java

@@ -5,21 +5,27 @@
 import org.junit.jupiter.api.Test;
 
 import edu.hm.hafner.coverage.Coverage.CoverageBuilder;
+import edu.hm.hafner.util.SerializableTest;
 
 import nl.jqno.equalsverifier.EqualsVerifier;
 import nl.jqno.equalsverifier.api.EqualsVerifierApi;
 import nl.jqno.equalsverifier.api.SingleTypeEqualsVerifierApi;
 
 import static edu.hm.hafner.coverage.assertions.Assertions.*;
 
-abstract class AbstractNodeTest {
+abstract class AbstractNodeTest extends SerializableTest<Node> {
     private static final String NAME = "Node Name";
     private static final String CHILD = "Child";
     private static final Coverage MUTATION_COVERAGE = new CoverageBuilder().setMetric(Metric.MUTATION)
             .setCovered(5)
             .setMissed(10)
             .build();
 
+    @Override
+    protected Node createSerializable() {
+        return createNode("Serialized");
+    }
+
     abstract Metric getMetric();
 
     abstract Node createNode(String name);

src/test/java/edu/hm/hafner/coverage/FileNodeTest.java

@@ -1,5 +1,6 @@
 package edu.hm.hafner.coverage;
 
+import org.apache.commons.lang3.StringUtils;
 import org.junit.jupiter.api.Test;
 
 import edu.hm.hafner.util.TreeString;
@@ -19,7 +20,6 @@ Metric getMetric() {
     void configureEqualsVerifier(final EqualsVerifierApi<? extends Node> verifier) {
         verifier.withPrefabValues(TreeString.class, TreeString.valueOf("src"), TreeString.valueOf("test"))
                 .suppress(Warning.NONFINAL_FIELDS);
-
     }
 
     @Override
@@ -66,4 +66,13 @@ void shouldGetFilePath() {
         assertThat(file.matches(Metric.FILE, otherPath.hashCode())).isTrue();
         assertThat(file.matches(Metric.FILE, "wrong".hashCode())).isFalse();
     }
+
+    @Test
+    void shouldReadOldVersion() {
+        byte[] restored = readAllBytes("version-0.21.0.ser");
+
+        var serializable = (FileNode)createSerializable();
+        serializable.setRelativePath(TreeString.valueOf(StringUtils.EMPTY));
+        assertThatRestoredInstanceEqualsOriginalInstance(serializable, restore(restored));
+    }
 }