[Java] Add support for shared strings on FlatBufferBuilder. (#6012)

Added a method FlatBufferBuilder::createSharedString that
enable string sharing for building messages on java.

The shared pool will only contains strings inserted by
this methods.

java/com/google/flatbuffers/FlatBufferBuilder.java

@@ -22,6 +22,9 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.nio.*;
 import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+import java.lang.Integer;
 
 /// @file
 /// @addtogroup flatbuffers_java_api
@@ -47,6 +50,7 @@ public class FlatBufferBuilder {
     boolean force_defaults = false;   // False omits default values from the serialized data.
     ByteBufferFactory bb_factory;     // Factory for allocating the internal buffer
     final Utf8 utf8;                  // UTF-8 encoder to use
+    Map<String, Integer> string_pool; // map used to cache shared strings.
     /// @endcond
 
     /**
@@ -147,6 +151,9 @@ public class FlatBufferBuilder {
         object_start = 0;
         num_vtables = 0;
         vector_num_elems = 0;
+        if (string_pool != null) {
+            string_pool.clear();
+        }
         return this;
     }
 
@@ -224,6 +231,9 @@ public class FlatBufferBuilder {
         object_start = 0;
         num_vtables = 0;
         vector_num_elems = 0;
+        if (string_pool != null) {
+            string_pool.clear();
+        }
     }
 
     /**
@@ -527,6 +537,37 @@ public class FlatBufferBuilder {
         return createVectorOfTables(offsets);
     }
 
+    /**
+    * Encode the String `s` in the buffer using UTF-8. If a String with
+    * this exact contents has already been serialized using this method,
+    * instead simply returns the offset of the existing String.
+    *
+    * Usage of the method will incur into additional allocations,
+    * so it is advisable to use it only when it is known upfront that
+    * your message will have several repeated strings.
+    *
+    * @param s The String to encode.
+    * @return The offset in the buffer where the encoded String starts.
+    */
+    public int createSharedString(String s) {
+
+        if (string_pool == null) {
+            string_pool = new HashMap<>();
+            int offset = createString(s);
+            string_pool.put(s, offset);
+            return offset;
+
+        }
+
+        Integer offset = string_pool.get(s);
+
+        if(offset == null) {
+            offset = createString(s);
+            string_pool.put(s, offset);
+        }
+        return offset;
+    }
+
    /**
     * Encode the string `s` in the buffer using UTF-8.  If {@code s} is
     * already a {@link CharBuffer}, this method is allocation free.

tests/JavaTest.java

@@ -23,7 +23,9 @@ import java.io.*;
 import java.math.BigInteger;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
+import java.nio.CharBuffer;
 import java.nio.channels.FileChannel;
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
@@ -101,6 +103,8 @@ class JavaTest {
 
         TestVectorOfBytes();
 
+        TestSharedStringPool();
+
         System.out.println("FlatBuffers test: completed successfully");
     }
 
@@ -1216,6 +1220,15 @@ class JavaTest {
         TestEq(monsterObject8.inventoryLength(), 2048);
     }
 
+    static void TestSharedStringPool() {
+        FlatBufferBuilder fb = new FlatBufferBuilder(1);
+        String testString = "My string";
+        int offset = fb.createSharedString(testString);
+        for (int i=0; i< 10; i++) {
+            TestEq(offset, fb.createSharedString(testString));
+        }
+    }
+
     static <T> void TestEq(T a, T b) {
         if (!a.equals(b)) {
             System.out.println("" + a.getClass().getName() + " " + b.getClass().getName());

tests/KotlinTest.kt

@@ -76,6 +76,8 @@ class KotlinTest {
 
         TestVectorOfUnions()
 
+        TestSharedStringPool()
+
         println("FlatBuffers test: completed successfully")
     }
 
@@ -456,5 +458,14 @@ class KotlinTest {
 
         assert((movie.characters(Attacker(), 0) as Attacker).swordAttackDamage == swordAttackDamage)
     }
-}
+
+    fun TestSharedStringPool() {
+        val fb = FlatBufferBuilder(1);
+        val testString = "My string";
+        val offset = fb.createSharedString(testString);
+        for (i in 0..10) {
+            assert(offset == fb.createSharedString(testString));
         }
+    }
+  }
+}