[Java] Add support for DTO generation.

This commit adds support for generating SBE DTOs in Java.

Unlike the C# and C++ support, there isn't a natural candidate for
the idiomatic representation of optional fields, e.g., `Optional<T>` is
only meant for use as a return type. Therefore, the mapping between Java
types and the encoding is simpler than in these other languages.

In a future commit, I will extend the property-based tests to cover Java
DTOs.

Author: ZachBray

build.gradle

@@ -280,6 +280,7 @@ project(':sbe-tool') {
 
     compileGeneratedJava {
         dependsOn 'generateTestCodecs'
+        dependsOn 'generateTestDtos'
         classpath += sourceSets.main.runtimeClasspath
     }
 
@@ -344,6 +345,21 @@ project(':sbe-tool') {
                 'src/test/resources/field-order-check-schema.xml']
     }
 
+    tasks.register('generateTestDtos', JavaExec) {
+        dependsOn 'compileJava'
+        mainClass.set('uk.co.real_logic.sbe.SbeTool')
+        classpath = sourceSets.main.runtimeClasspath
+        systemProperties(
+                'sbe.output.dir': generatedDir,
+                'sbe.target.language': 'java',
+                'sbe.validation.stop.on.error': 'true',
+                'sbe.validation.xsd': validationXsdPath,
+                'sbe.generate.precedence.checks': 'true',
+                'sbe.java.precedence.checks.property.name': 'sbe.enable.test.precedence.checks',
+                'sbe.java.generate.dtos': 'true')
+        args = ['src/test/resources/example-extension-schema.xml']
+    }
+
     jar {
         manifest.attributes(
             'Specification-Title': 'Simple Binary Encoding',

sbe-tool/src/main/java/uk/co/real_logic/sbe/SbeTool.java

@@ -195,7 +195,12 @@ public class SbeTool
     /**
      * Should generate C++ DTOs. Defaults to false.
      */
-    public static final String GENERATE_CPP_DTOS = "sbe.cpp.generate.dtos";
+    public static final String CPP_GENERATE_DTOS = "sbe.cpp.generate.dtos";
+
+    /**
+     * Should generate Java DTOs. Defaults to false.
+     */
+    public static final String JAVA_GENERATE_DTOS = "sbe.java.generate.dtos";
 
     /**
      * Configuration option used to manage sinceVersion based transformations. When set, parsed schemas will be

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/TargetCodeGeneratorLoader.java

@@ -23,6 +23,7 @@
 import uk.co.real_logic.sbe.generation.cpp.NamespaceOutputManager;
 import uk.co.real_logic.sbe.generation.golang.GolangGenerator;
 import uk.co.real_logic.sbe.generation.golang.GolangOutputManager;
+import uk.co.real_logic.sbe.generation.java.JavaDtoGenerator;
 import uk.co.real_logic.sbe.generation.java.JavaGenerator;
 import uk.co.real_logic.sbe.generation.java.JavaOutputManager;
 import uk.co.real_logic.sbe.generation.rust.RustGenerator;
@@ -47,7 +48,9 @@ public enum TargetCodeGeneratorLoader implements TargetCodeGenerator
          */
         public CodeGenerator newInstance(final Ir ir, final String outputDir)
         {
-            return new JavaGenerator(
+            final JavaOutputManager outputManager = new JavaOutputManager(outputDir, ir.applicableNamespace());
+
+            final JavaGenerator codecGenerator = new JavaGenerator(
                 ir,
                 System.getProperty(JAVA_ENCODING_BUFFER_TYPE, JAVA_DEFAULT_ENCODING_BUFFER_TYPE),
                 System.getProperty(JAVA_DECODING_BUFFER_TYPE, JAVA_DEFAULT_DECODING_BUFFER_TYPE),
@@ -56,7 +59,18 @@ public CodeGenerator newInstance(final Ir ir, final String outputDir)
                 "true".equals(System.getProperty(DECODE_UNKNOWN_ENUM_VALUES)),
                 "true".equals(System.getProperty(TYPES_PACKAGE_OVERRIDE)),
                 precedenceChecks(),
-                new JavaOutputManager(outputDir, ir.applicableNamespace()));
+                outputManager);
+
+            final JavaDtoGenerator dtoGenerator = new JavaDtoGenerator(ir, outputManager);
+
+            final CodeGenerator combinedGenerator = () ->
+            {
+                codecGenerator.generate();
+                dtoGenerator.generate();
+            };
+
+            final boolean generateDtos = "true".equals(System.getProperty(JAVA_GENERATE_DTOS));
+            return generateDtos ? combinedGenerator : codecGenerator;
         }
     },
 
@@ -97,7 +111,7 @@ public CodeGenerator newInstance(final Ir ir, final String outputDir)
                 dtoGenerator.generate();
             };
 
-            final boolean generateDtos = "true".equals(System.getProperty(GENERATE_CPP_DTOS));
+            final boolean generateDtos = "true".equals(System.getProperty(CPP_GENERATE_DTOS));
             return generateDtos ? combinedGenerator : codecGenerator;
         }
     },

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppDtos.java

@@ -22,7 +22,7 @@
 import uk.co.real_logic.sbe.ir.Ir;
 
 /**
- * {@link CodeGenerator} factory for CSharp DTOs.
+ * {@link CodeGenerator} factory for C++ DTOs.
  */
 public class CppDtos implements TargetCodeGenerator
 {