general changes to support flow file validation (DAT-10588) (#2986)

add createIfNull(Map) method to CollectionUtil
expose separate validate() method in CommandScope
add constructor to CommandValidationException(Throwable)

liquibase-core/src/main/java/liquibase/command/CommandScope.java

@@ -3,6 +3,7 @@
 import liquibase.Scope;
 import liquibase.configuration.*;
 import liquibase.exception.CommandExecutionException;
+import liquibase.exception.CommandValidationException;
 import liquibase.util.StringUtil;
 
 import java.io.OutputStream;
@@ -131,11 +132,7 @@ public CommandScope setOutput(OutputStream outputStream) {
         return this;
     }
 
-    /**
-     * Executes the command in this scope, and returns the results.
-     */
-    public CommandResults execute() throws CommandExecutionException {
-        CommandResultsBuilder resultsBuilder = new CommandResultsBuilder(this, outputStream);
+    public void validate() throws CommandValidationException {
         for (ConfigurationValueProvider provider : Scope.getCurrentScope().getSingleton(LiquibaseConfiguration.class).getProviders()) {
             provider.validate(this);
         }
@@ -151,6 +148,15 @@ public CommandResults execute() throws CommandExecutionException {
         for (CommandStep step : pipeline) {
             step.validate(this);
         }
+    }
+
+    /**
+     * Executes the command in this scope, and returns the results.
+     */
+    public CommandResults execute() throws CommandExecutionException {
+        CommandResultsBuilder resultsBuilder = new CommandResultsBuilder(this, outputStream);
+        final List<CommandStep> pipeline = commandDefinition.getPipeline();
+        validate();
         try {
             for (CommandStep command : pipeline) {
                 command.run(resultsBuilder);

liquibase-core/src/main/java/liquibase/exception/CommandValidationException.java

@@ -13,6 +13,10 @@ public CommandValidationException(String message) {
         super(message);
     }
 
+    public CommandValidationException(Throwable cause) {
+        super(cause);
+    }
+
     public CommandValidationException(String argument, String message, Throwable cause) {
         super(buildMessage(argument, message), cause);
     }

liquibase-core/src/main/java/liquibase/util/CollectionUtil.java

@@ -78,7 +78,7 @@ public static <T> T[] createIfNull(T[] arguments) {
     }
 
     /**
-     * Returns a new empty set if the passed array is null.
+     * Returns a new empty set if the passed set is null.
      */
     public static <T> Set<T> createIfNull(Set<T> currentValue) {
         if (currentValue == null) {
@@ -88,6 +88,17 @@ public static <T> Set<T> createIfNull(Set<T> currentValue) {
         }
     }
 
+    /**
+     * Returns a new empty map if the passed map is null.
+     */
+    public static <T, E> Map<T, E> createIfNull(Map<T, E> currentValue) {
+        if (currentValue == null) {
+            return new HashMap<>();
+        } else {
+            return currentValue;
+        }
+    }
+
     /**
      * Converts a set of nested maps (like from yaml/json) into a flat map with dot-separated properties
      */