initial changes to support size prefixed buffers in Java

java/com/google/flatbuffers/FlatBufferBuilder.java

@@ -816,14 +816,55 @@ public class FlatBufferBuilder {
      * Finalize a buffer, pointing to the given `root_table`.
      *
      * @param root_table An offset to be added to the buffer.
+     * @param size_prefix Whether to prefix the size to the buffer.
      */
-    public void finish(int root_table) {
+    protected void finish(int root_table, boolean size_prefix) {
-        prep(minalign, SIZEOF_INT);
+        prep(minalign, SIZEOF_INT + (size_prefix ? SIZEOF_INT : 0));
         addOffset(root_table);
+        if (size_prefix) {
+            addInt(bb.capacity() - space);
+        }
         bb.position(space);
         finished = true;
     }
 
+    /**
+     * Finalize a buffer, pointing to the given `root_table`.
+     *
+     * @param root_table An offset to be added to the buffer.
+     */
+    public void finish(int root_table) {
+        finish(root_table, false);
+    }
+
+    /**
+     * Finalize a buffer, pointing to the given `root_table`.+, with the size prefixed.
+     *
+     * @param root_table An offset to be added to the buffer.
+     */
+    public void finishSizePrefixed(int root_table) {
+        finish(root_table, true);
+    }
+
+    /**
+     * Finalize a buffer, pointing to the given `root_table`.
+     *
+     * @param root_table An offset to be added to the buffer.
+     * @param file_identifier A FlatBuffer file identifier to be added to the buffer before
+     * `root_table`.
+     * @param size_prefix Whether to prefix the size to the buffer.
+     */
+    protected void finish(int root_table, String file_identifier, boolean size_prefix) {
+        prep(minalign, SIZEOF_INT + FILE_IDENTIFIER_LENGTH + (size_prefix ? SIZEOF_INT : 0));
+        if (file_identifier.length() != FILE_IDENTIFIER_LENGTH)
+            throw new AssertionError("FlatBuffers: file identifier must be length " +
+                                     FILE_IDENTIFIER_LENGTH);
+        for (int i = FILE_IDENTIFIER_LENGTH - 1; i >= 0; i--) {
+            addByte((byte)file_identifier.charAt(i));
+        }
+        finish(root_table, size_prefix);
+    }
+
     /**
      * Finalize a buffer, pointing to the given `root_table`.
      *
@@ -832,14 +873,18 @@ public class FlatBufferBuilder {
      * `root_table`.
      */
     public void finish(int root_table, String file_identifier) {
-        prep(minalign, SIZEOF_INT + FILE_IDENTIFIER_LENGTH);
+        finish(root_table, file_identifier, false);
-        if (file_identifier.length() != FILE_IDENTIFIER_LENGTH)
+    }
-            throw new AssertionError("FlatBuffers: file identifier must be length " +
+
-                                     FILE_IDENTIFIER_LENGTH);
+    /**
-        for (int i = FILE_IDENTIFIER_LENGTH - 1; i >= 0; i--) {
+     * Finalize a buffer, pointing to the given `root_table`, with the size prefixed.
-            addByte((byte)file_identifier.charAt(i));
+     *
-        }
+     * @param root_table An offset to be added to the buffer.
-        finish(root_table);
+     * @param file_identifier A FlatBuffer file identifier to be added to the buffer before
+     * `root_table`.
+     */
+    public void finishSizePrefixed(int root_table, String file_identifier) {
+        finish(root_table, file_identifier, true);
     }
 
     /**

src/idl_gen_general.cpp

@@ -828,6 +828,30 @@ void GenStruct(StructDef &struct_def, std::string *code_ptr) {
     code += ") + _bb.";
     code += lang_.get_bb_position;
     code += ", _bb)); }\n";
+
+    // recreate both methods for the prefixed size version
+    std::string ps_method_name = FunctionStart('G') + "etSizePrefixedRootAs" +
+                                 struct_def.name;
+    std::string ps_method_signature = "  public static " + struct_def.name + " " +
+                                      ps_method_name;
+
+    // create convenience method that doesn't require an existing object
+    code += ps_method_signature + "(ByteBuffer _psbb) ";
+    code += "{ return " + ps_method_name + "(_psbb, new " + struct_def.name+ "()); }\n";
+
+    // TODO this part needs a C# equivalent
+    // use a slice that skips the size, then proceed as normal
+    code += ps_method_signature + "(ByteBuffer _psbb, " + struct_def.name + " obj) { ";
+    code += "ByteBuffer _bb = _psbb.slice(); ";
+    code += "_bb.position(4); ";
+    code += "return " + method_name + "(_bb, obj); }\n";
+
+    // method that returns the size for a size prefixed buffer
+    code += "  public static int " + FunctionStart('G') + "etSizePrefix(ByteBuffer _bb) { ";
+    code += lang_.set_bb_byteorder;
+    code += "return _bb." + FunctionStart('G') + "etInt(_bb.";
+    code += lang_.get_bb_position + "); }\n";
+
     if (parser_.root_struct_def_ == &struct_def) {
       if (parser_.file_identifier_.length()) {
         // Check if a buffer has the identifier.
@@ -1305,18 +1329,22 @@ void GenStruct(StructDef &struct_def, std::string *code_ptr) {
     }
     code += "    return " + GenOffsetConstruct(struct_def, "o") + ";\n  }\n";
     if (parser_.root_struct_def_ == &struct_def) {
-      code += "  public static void ";
+      std::string size_prefix[] = { "", "SizePrefixed" };
-      code += FunctionStart('F') + "inish" + struct_def.name;
+      for (int i = 0; i < 2; ++i) {
-      code += "Buffer(FlatBufferBuilder builder, " + GenOffsetType(struct_def);
+        code += "  public static void ";
-      code += " offset) {";
+        code += FunctionStart('F') + "inish" + size_prefix[i] + struct_def.name;
-      code += " builder." + FunctionStart('F') + "inish(offset";
+        code += "Buffer(FlatBufferBuilder builder, " + GenOffsetType(struct_def);
-      if (lang_.language == IDLOptions::kCSharp) {
+        code += " offset) {";
-        code += ".Value";
+        code += " builder." + FunctionStart('F') + "inish" + size_prefix[i] +
-      }
+                "(offset";
+        if (lang_.language == IDLOptions::kCSharp) {
+          code += ".Value";
+        }
 
-      if (parser_.file_identifier_.length())
+        if (parser_.file_identifier_.length())
-        code += ", \"" + parser_.file_identifier_ + "\"";
+          code += ", \"" + parser_.file_identifier_ + "\"";
-      code += "); }\n";
+        code += "); }\n";
+      }
     }
   }
   // Only generate key compare function for table,