Merge pull request #3305 from mikecirioli/JENKINS-49588

[JENKINS-49588] add readResolve method for 'usages' in Fingerprint.java

core/src/main/java/hudson/model/Fingerprint.java

@@ -866,7 +866,7 @@ public class Fingerprint implements ModelObject, Saveable {
     /**
      * Range of builds that use this file keyed by a job full name.
      */
-    private final Hashtable<String,RangeSet> usages = new Hashtable<String,RangeSet>();
+    private Hashtable<String,RangeSet> usages = new Hashtable<String,RangeSet>();
 
     PersistedList<FingerprintFacet> facets = new PersistedList<FingerprintFacet>(this);
 
@@ -1029,6 +1029,14 @@ public class Fingerprint implements ModelObject, Saveable {
         save();
     }
 
+    // JENKINS-49588
+    protected Object readResolve() {
+        if (usages == null) {
+            usages = new Hashtable<String,RangeSet>();
+        }
+        return this;
+    }
+
     void addWithoutSaving(@Nonnull String jobFullName, int n) {
         synchronized(usages) { // TODO why not synchronized (this) like some, though not all, other accesses?
             RangeSet r = usages.get(jobFullName);

core/src/test/java/hudson/model/FingerprintTest.java

@@ -222,6 +222,13 @@ public class FingerprintTest {
                 Fingerprint.load(new File(FingerprintTest.class.getResource("fingerprint.xml").toURI())).toString());
     }
 
+    @Test public void loadFingerprintWithoutUsages() throws Exception {
+        Fingerprint fp = Fingerprint.load(new File(FingerprintTest.class.getResource("fingerprintWithoutUsages.xml").toURI()));
+        assertNotNull(fp);
+        assertEquals("test:jenkinsfile-example-1.0-SNAPSHOT.jar", fp.getFileName());
+        assertNotNull(fp.getUsages());
+    }
+
     @Test public void roundTrip() throws Exception {
         Fingerprint f = new Fingerprint(new Fingerprint.BuildPtr("foo", 13), "stuff&more.jar", SOME_MD5);
         f.addWithoutSaving("some", 1);

core/src/test/resources/hudson/model/fingerprintWithoutUsages.xml

+<?xml version='1.0' encoding='UTF-8'?>
+<fingerprint>
+  <timestamp>2018-02-21 16:33:12.291 UTC</timestamp>
+  <original>
+    <name>maven1/test:jenkinsfile-example</name>
+    <number>4</number>
+  </original>
+  <md5sum>f003726dfd1d07868e3c1ab5aab0682d</md5sum>
+  <fileName>test:jenkinsfile-example-1.0-SNAPSHOT.jar</fileName>
+  <facets/>
+</fingerprint>