[MSHARED-1382] support a possibility to disable snapshots update.

src/main/java/org/apache/maven/shared/invoker/DefaultInvocationRequest.java

@@ -63,7 +63,7 @@ public class DefaultInvocationRequest implements InvocationRequest {
 
     private boolean showErrors;
 
-    private boolean updateSnapshots;
+    private UpdateSnapshotsPolicy updateSnapshotsPolicy = UpdateSnapshotsPolicy.DEFAULT;
 
     private boolean shellEnvironmentInherited = true;
 
@@ -227,7 +227,11 @@ public boolean isShowErrors() {
      * @return a boolean.
      */
     public boolean isUpdateSnapshots() {
-        return updateSnapshots;
+        return updateSnapshotsPolicy == UpdateSnapshotsPolicy.ALWAYS;
+    }
+
+    public UpdateSnapshotsPolicy getUpdateSnapshotsPolicy() {
+        return updateSnapshotsPolicy;
     }
 
     /**
@@ -330,7 +334,12 @@ public InvocationRequest setShowErrors(boolean showErrors) {
 
     /** {@inheritDoc} */
     public InvocationRequest setUpdateSnapshots(boolean updateSnapshots) {
-        this.updateSnapshots = updateSnapshots;
+        return setUpdateSnapshotsPolicy(updateSnapshots ? UpdateSnapshotsPolicy.ALWAYS : UpdateSnapshotsPolicy.DEFAULT);
+    }
+
+    @Override
+    public InvocationRequest setUpdateSnapshotsPolicy(UpdateSnapshotsPolicy policy) {
+        this.updateSnapshotsPolicy = policy;
         return this;
     }
 

src/main/java/org/apache/maven/shared/invoker/InvocationRequest.java

@@ -51,10 +51,19 @@ public interface InvocationRequest {
      * Indicates whether Maven should enforce an update check for plugins and snapshots. By default, no update check is
      * performed.
      *
-     * @return <code>true</code> if plugins and snapshots should be updated, <code>false</code> otherwise.
+     * @return <code>true</code> if plugins and snapshots must be updated, <code>false</code> otherwise.
+     *
+     * @see #getUpdateSnapshotsPolicy() which provides a richer variety of the update snapshots policy values.
      */
     boolean isUpdateSnapshots();
 
+    /**
+     * Indicates the update snapshots policy.
+     * @return the update snapshots policy.
+     * @see UpdateSnapshotsPolicy
+     */
+    UpdateSnapshotsPolicy getUpdateSnapshotsPolicy();
+
     /**
      * Gets the recursion behavior of a reactor invocation. By default, Maven will recursive the build into sub modules.
      *
@@ -451,12 +460,23 @@ enum CheckSumPolicy {
      * Specifies whether Maven should enforce an update check for plugins and snapshots. Equivalent of {@code -U} and
      * {@code --update-snapshots}
      *
-     * @param updateSnapshots <code>true</code> if plugins and snapshots should be updated, <code>false</code>
+     * @param updateSnapshots <code>true</code> if plugins and snapshots must be updated, <code>false</code>
      *            otherwise.
      * @return This invocation request.
+     *
+     * @see #setUpdateSnapshotsPolicy(UpdateSnapshotsPolicy) which provides a richer variety of the update snapshots policy values.
      */
     InvocationRequest setUpdateSnapshots(boolean updateSnapshots);
 
+    /**
+     * Specify the Maven update snapshots policy
+     * @param policy the policy to be set
+     * @return This invocation request.
+     *
+     * @see UpdateSnapshotsPolicy
+     */
+    InvocationRequest setUpdateSnapshotsPolicy(UpdateSnapshotsPolicy policy);
+
     /**
      * Sets the failure mode of the Maven invocation. Equivalent of {@code -ff} and {@code --fail-fast}, {@code -fae}
      * and {@code --fail-at-end}, {@code -fn} and {@code --fail-never}

src/main/java/org/apache/maven/shared/invoker/MavenCommandLineBuilder.java

@@ -408,10 +408,14 @@ protected void setFlags(InvocationRequest request, Commandline cli) {
             cli.createArg().setValue("-o");
         }
 
-        if (request.isUpdateSnapshots()) {
+        if (request.getUpdateSnapshotsPolicy() == UpdateSnapshotsPolicy.ALWAYS) {
             cli.createArg().setValue("-U");
         }
 
+        if (request.getUpdateSnapshotsPolicy() == UpdateSnapshotsPolicy.NEVER) {
+            cli.createArg().setValue("-nsu");
+        }
+
         if (!request.isRecursive()) {
             cli.createArg().setValue("-N");
         }

src/main/java/org/apache/maven/shared/invoker/UpdateSnapshotsPolicy.java

@@ -0,0 +1,39 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.maven.shared.invoker;
+
+/**
+ * Define how Maven should update snapshots.
+ */
+public enum UpdateSnapshotsPolicy {
+    /**
+     * Request Maven to always update snapshots.
+     */
+    ALWAYS,
+
+    /**
+     * Don't control Maven policy on snapshots updates.
+     */
+    DEFAULT,
+
+    /**
+     * Prevent Maven updating snapshots.
+     */
+    NEVER
+}

src/test/java/org/apache/maven/shared/invoker/MavenCommandLineBuilderTest.java

@@ -299,6 +299,33 @@ public void testShouldSetUpdateSnapshotsFlagFromRequest() {
         assertArgumentsPresent(cli, Collections.singleton("-U"));
     }
 
+    // JUnit5: test methods don't need to be public
+    @Test
+    void testShouldSetUpdateSnapshotsPolicyAlwaysFromRequest() {
+        mclb.setFlags(newRequest().setUpdateSnapshotsPolicy(UpdateSnapshotsPolicy.ALWAYS), cli);
+
+        assertArgumentsPresent(cli, Collections.singleton("-U"));
+        assertArgumentsNotPresent(cli, Collections.singleton("-nsu"));
+    }
+
+    @Test
+    void testShouldSetUpdateSnapshotsPolicyDefaultFromRequest() {
+        mclb.setFlags(newRequest().setUpdateSnapshotsPolicy(UpdateSnapshotsPolicy.DEFAULT), cli);
+
+        Set<String> args = new HashSet<>();
+        args.add("-U");
+        args.add("-nsu");
+        assertArgumentsNotPresent(cli, args);
+    }
+
+    @Test
+    void testShouldSetUpdateSnapshotsPolicyNeverFromRequest() {
+        mclb.setFlags(newRequest().setUpdateSnapshotsPolicy(UpdateSnapshotsPolicy.NEVER), cli);
+
+        assertArgumentsPresent(cli, Collections.singleton("-nsu"));
+        assertArgumentsNotPresent(cli, Collections.singleton("-U"));
+    }
+
     @Test
     public void testShouldSetDebugFlagFromRequest() {