Implement JSON Path for JsonTreeReader.

Author: JakeWharton

gson/src/main/java/com/google/gson/internal/bind/JsonTreeReader.java

@@ -25,9 +25,7 @@
 import com.google.gson.stream.JsonToken;
 import java.io.IOException;
 import java.io.Reader;
-import java.util.ArrayList;
 import java.util.Iterator;
-import java.util.List;
 import java.util.Map;
 
 /**
@@ -47,35 +45,57 @@ public final class JsonTreeReader extends JsonReader {
   };
   private static final Object SENTINEL_CLOSED = new Object();
 
-  private final List<Object> stack = new ArrayList<Object>();
+  /*
+   * The nesting stack. Using a manual array rather than an ArrayList saves 20%.
+   */
+  private Object[] stack = new Object[32];
+  private int stackSize = 0;
+
+  /*
+   * The path members. It corresponds directly to stack: At indices where the
+   * stack contains an object (EMPTY_OBJECT, DANGLING_NAME or NONEMPTY_OBJECT),
+   * pathNames contains the name at this scope. Where it contains an array
+   * (EMPTY_ARRAY, NONEMPTY_ARRAY) pathIndices contains the current index in
+   * that array. Otherwise the value is undefined, and we take advantage of that
+   * by incrementing pathIndices when doing so isn't useful.
+   */
+  private String[] pathNames = new String[32];
+  private int[] pathIndices = new int[32];
 
   public JsonTreeReader(JsonElement element) {
     super(UNREADABLE_READER);
-    stack.add(element);
+    push(element);
   }
 
   @Override public void beginArray() throws IOException {
     expect(JsonToken.BEGIN_ARRAY);
     JsonArray array = (JsonArray) peekStack();
-    stack.add(array.iterator());
+    push(array.iterator());
+    pathIndices[stackSize - 1] = 0;
   }
 
   @Override public void endArray() throws IOException {
     expect(JsonToken.END_ARRAY);
     popStack(); // empty iterator
     popStack(); // array
+    if (stackSize > 0) {
+      pathIndices[stackSize - 1]++;
+    }
   }
 
   @Override public void beginObject() throws IOException {
     expect(JsonToken.BEGIN_OBJECT);
     JsonObject object = (JsonObject) peekStack();
-    stack.add(object.entrySet().iterator());
+    push(object.entrySet().iterator());
   }
 
   @Override public void endObject() throws IOException {
     expect(JsonToken.END_OBJECT);
     popStack(); // empty iterator
     popStack(); // object
+    if (stackSize > 0) {
+      pathIndices[stackSize - 1]++;
+    }
   }
 
   @Override public boolean hasNext() throws IOException {
@@ -84,19 +104,19 @@ public JsonTreeReader(JsonElement element) {
   }
 
   @Override public JsonToken peek() throws IOException {
-    if (stack.isEmpty()) {
+    if (stackSize == 0) {
       return JsonToken.END_DOCUMENT;
     }
 
     Object o = peekStack();
     if (o instanceof Iterator) {
-      boolean isObject = stack.get(stack.size() - 2) instanceof JsonObject;
+      boolean isObject = stack[stackSize - 2] instanceof JsonObject;
       Iterator<?> iterator = (Iterator<?>) o;
       if (iterator.hasNext()) {
         if (isObject) {
           return JsonToken.NAME;
         } else {
-          stack.add(iterator.next());
+          push(iterator.next());
           return peek();
         }
       } else {
@@ -127,89 +147,121 @@ public JsonTreeReader(JsonElement element) {
   }
 
   private Object peekStack() {
-    return stack.get(stack.size() - 1);
+    return stack[stackSize - 1];
   }
 
   private Object popStack() {
-    return stack.remove(stack.size() - 1);
+    Object result = stack[--stackSize];
+    stack[stackSize] = null;
+    return result;
   }
 
   private void expect(JsonToken expected) throws IOException {
     if (peek() != expected) {
-      throw new IllegalStateException("Expected " + expected + " but was " + peek());
+      throw new IllegalStateException(
+          "Expected " + expected + " but was " + peek() + locationString());
     }
   }
 
   @Override public String nextName() throws IOException {
     expect(JsonToken.NAME);
     Iterator<?> i = (Iterator<?>) peekStack();
     Map.Entry<?, ?> entry = (Map.Entry<?, ?>) i.next();
-    stack.add(entry.getValue());
-    return (String) entry.getKey();
+    String result = (String) entry.getKey();
+    pathNames[stackSize - 1] = result;
+    push(entry.getValue());
+    return result;
   }
 
   @Override public String nextString() throws IOException {
     JsonToken token = peek();
     if (token != JsonToken.STRING && token != JsonToken.NUMBER) {
-      throw new IllegalStateException("Expected " + JsonToken.STRING + " but was " + token);
+      throw new IllegalStateException(
+          "Expected " + JsonToken.STRING + " but was " + token + locationString());
+    }
+    String result = ((JsonPrimitive) popStack()).getAsString();
+    if (stackSize > 0) {
+      pathIndices[stackSize - 1]++;
     }
-    return ((JsonPrimitive) popStack()).getAsString();
+    return result;
   }
 
   @Override public boolean nextBoolean() throws IOException {
     expect(JsonToken.BOOLEAN);
-    return ((JsonPrimitive) popStack()).getAsBoolean();
+    boolean result = ((JsonPrimitive) popStack()).getAsBoolean();
+    if (stackSize > 0) {
+      pathIndices[stackSize - 1]++;
+    }
+    return result;
   }
 
   @Override public void nextNull() throws IOException {
     expect(JsonToken.NULL);
     popStack();
+    if (stackSize > 0) {
+      pathIndices[stackSize - 1]++;
+    }
   }
 
   @Override public double nextDouble() throws IOException {
     JsonToken token = peek();
     if (token != JsonToken.NUMBER && token != JsonToken.STRING) {
-      throw new IllegalStateException("Expected " + JsonToken.NUMBER + " but was " + token);
+      throw new IllegalStateException(
+          "Expected " + JsonToken.NUMBER + " but was " + token + locationString());
     }
     double result = ((JsonPrimitive) peekStack()).getAsDouble();
     if (!isLenient() && (Double.isNaN(result) || Double.isInfinite(result))) {
       throw new NumberFormatException("JSON forbids NaN and infinities: " + result);
     }
     popStack();
+    if (stackSize > 0) {
+      pathIndices[stackSize - 1]++;
+    }
     return result;
   }
 
   @Override public long nextLong() throws IOException {
     JsonToken token = peek();
     if (token != JsonToken.NUMBER && token != JsonToken.STRING) {
-      throw new IllegalStateException("Expected " + JsonToken.NUMBER + " but was " + token);
+      throw new IllegalStateException(
+          "Expected " + JsonToken.NUMBER + " but was " + token + locationString());
     }
     long result = ((JsonPrimitive) peekStack()).getAsLong();
     popStack();
+    if (stackSize > 0) {
+      pathIndices[stackSize - 1]++;
+    }
     return result;
   }
 
   @Override public int nextInt() throws IOException {
     JsonToken token = peek();
     if (token != JsonToken.NUMBER && token != JsonToken.STRING) {
-      throw new IllegalStateException("Expected " + JsonToken.NUMBER + " but was " + token);
+      throw new IllegalStateException(
+          "Expected " + JsonToken.NUMBER + " but was " + token + locationString());
     }
     int result = ((JsonPrimitive) peekStack()).getAsInt();
     popStack();
+    if (stackSize > 0) {
+      pathIndices[stackSize - 1]++;
+    }
     return result;
   }
 
   @Override public void close() throws IOException {
-    stack.clear();
-    stack.add(SENTINEL_CLOSED);
+    stack = new Object[] { SENTINEL_CLOSED };
+    stackSize = 1;
   }
 
   @Override public void skipValue() throws IOException {
     if (peek() == JsonToken.NAME) {
       nextName();
+      pathNames[stackSize - 2] = "null";
     } else {
       popStack();
+      pathNames[stackSize - 1] = "null";
     }
+    pathIndices[stackSize - 1]++;
   }
 
   @Override public String toString() {
@@ -220,7 +272,45 @@ public void promoteNameToValue() throws IOException {
     expect(JsonToken.NAME);
     Iterator<?> i = (Iterator<?>) peekStack();
     Map.Entry<?, ?> entry = (Map.Entry<?, ?>) i.next();
-    stack.add(entry.getValue());
-    stack.add(new JsonPrimitive((String)entry.getKey()));
+    push(entry.getValue());
+    push(new JsonPrimitive((String) entry.getKey()));
+  }
+
+  private void push(Object newTop) {
+    if (stackSize == stack.length) {
+      Object[] newStack = new Object[stackSize * 2];
+      int[] newPathIndices = new int[stackSize * 2];
+      String[] newPathNames = new String[stackSize * 2];
+      System.arraycopy(stack, 0, newStack, 0, stackSize);
+      System.arraycopy(pathIndices, 0, newPathIndices, 0, stackSize);
+      System.arraycopy(pathNames, 0, newPathNames, 0, stackSize);
+      stack = newStack;
+      pathIndices = newPathIndices;
+      pathNames = newPathNames;
+    }
+    stack[stackSize++] = newTop;
+  }
+
+  @Override public String getPath() {
+    StringBuilder result = new StringBuilder().append('$');
+    for (int i = 0; i < stackSize; i++) {
+      if (stack[i] instanceof JsonArray) {
+        if (stack[++i] instanceof Iterator) {
+          result.append('[').append(pathIndices[i]).append(']');
+        }
+      } else if (stack[i] instanceof JsonObject) {
+        if (stack[++i] instanceof Iterator) {
+          result.append('.');
+          if (pathNames[i] != null) {
+            result.append(pathNames[i]);
+          }
+        }
+      }
+    }
+    return result.toString();
+  }
+
+  private String locationString() {
+    return " at path " + getPath();
   }
 }