[rest] refactored the Stream2JSONInputStream to simplify the logic (#4099)

Signed-off-by: Jörg Sautter <joerg.sautter@gmx.net>

bundles/org.openhab.core.io.rest/src/main/java/org/openhab/core/io/rest/Stream2JSONInputStream.java

@@ -15,11 +15,15 @@ package org.openhab.core.io.rest;
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.OutputStream;
+import java.io.SequenceInputStream;
 import java.nio.charset.StandardCharsets;
+import java.util.Enumeration;
 import java.util.Iterator;
 import java.util.stream.Stream;
 
 import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.library.types.DateTimeType;
 
 import com.google.gson.Gson;
@@ -32,17 +36,14 @@ import com.google.gson.GsonBuilder;
  * nested collections JSON representation will be fully transformed into memory.
  *
  * @author Henning Treu - Initial contribution
+ * @author Jörg Sautter - Use as SequenceInputStream to simplify the logic
  */
 @NonNullByDefault
 public class Stream2JSONInputStream extends InputStream implements JSONInputStream {
 
-    private final Iterator<String> iterator;
+    private static final Gson GSON = new GsonBuilder().setDateFormat(DateTimeType.DATE_PATTERN_WITH_TZ_AND_MS).create();
 
-    private InputStream jsonElementStream;
-
-    private boolean firstIteratorElement;
-
-    private final Gson gson = new GsonBuilder().setDateFormat(DateTimeType.DATE_PATTERN_WITH_TZ_AND_MS).create();
+    private final InputStream stream;
 
     /**
      * Creates a new {@link Stream2JSONInputStream} backed by the given {@link Stream} source.
@@ -50,56 +51,68 @@ public class Stream2JSONInputStream extends InputStream implements JSONInputStre
      * @param source the {@link Stream} backing this input stream. Must not be null.
      */
     public Stream2JSONInputStream(Stream<?> source) {
-        iterator = source.map(e -> gson.toJson(e)).iterator();
-        jsonElementStream = new ByteArrayInputStream(new byte[0]);
-        firstIteratorElement = true;
+        Iterator<String> iterator = source.map(e -> GSON.toJson(e)).iterator();
+
+        Enumeration<InputStream> enumeration = new Enumeration<>() {
+            private boolean consumed = false;
+            private @Nullable InputStream next = toStream("[");
+
+            @Override
+            public boolean hasMoreElements() {
+                return next != null || iterator.hasNext();
+            }
+
+            @Override
+            public InputStream nextElement() {
+                InputStream is;
+
+                if (next != null) {
+                    is = next;
+                    if (!consumed && !iterator.hasNext()) {
+                        next = toStream("]");
+                        consumed = true;
+                    } else {
+                        next = null;
+                    }
+                    return is;
+                }
+
+                is = toStream(iterator.next());
+
+                if (iterator.hasNext()) {
+                    next = toStream(",");
+                } else {
+                    next = toStream("]");
+                    consumed = true;
+                }
+
+                return is;
+            }
+
+            private static InputStream toStream(String data) {
+                return new ByteArrayInputStream(data.getBytes(StandardCharsets.UTF_8));
+            }
+        };
+        stream = new SequenceInputStream(enumeration);
     }
 
     @Override
     public int read() throws IOException {
-        int result = jsonElementStream.read();
+        return stream.read();
+    }
 
-        if (result == -1) { // the current JSON element was completely streamed
-            if (finished()) { // we are done streaming the collection
-                return -1;
-            }
+    @Override
+    public int read(byte @Nullable [] b, int off, int len) throws IOException {
+        return stream.read(b, off, len);
+    }
 
-            fillBuffer(); // get the next element into a new jsonElementStream
-            result = jsonElementStream.read();
-        }
-
-        return result;
+    @Override
+    public long transferTo(OutputStream target) throws IOException {
+        return stream.transferTo(target);
     }
 
     @Override
     public void close() throws IOException {
-        jsonElementStream.close();
-    }
-
-    private void fillBuffer() {
-        String prefix;
-        if (firstIteratorElement) {
-            prefix = "[";
-            firstIteratorElement = false;
-        } else {
-            prefix = ",";
-        }
-
-        String entity = iterator.hasNext() ? iterator.next() : "";
-
-        String postfix = "";
-        if (!iterator.hasNext()) {
-            postfix = "]";
-        }
-
-        try {
-            jsonElementStream.close();
-        } catch (IOException e) {
-        }
-        jsonElementStream = new ByteArrayInputStream((prefix + entity + postfix).getBytes(StandardCharsets.UTF_8));
-    }
-
-    private boolean finished() {
-        return !firstIteratorElement && !iterator.hasNext();
+        stream.close();
     }
 }