ClientSerializationStreamReader.java

/*
 * Copyright 2008 Google Inc.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */
package com.google.gwt.user.client.rpc.impl;

import com.google.gwt.dev.jjs.SourceOrigin;
import com.google.gwt.dev.js.JsParser;
import com.google.gwt.dev.js.ast.JsArrayLiteral;
import com.google.gwt.dev.js.ast.JsBinaryOperation;
import com.google.gwt.dev.js.ast.JsBinaryOperator;
import com.google.gwt.dev.js.ast.JsBooleanLiteral;
import com.google.gwt.dev.js.ast.JsContext;
import com.google.gwt.dev.js.ast.JsExpression;
import com.google.gwt.dev.js.ast.JsInvocation;
import com.google.gwt.dev.js.ast.JsModVisitor;
import com.google.gwt.dev.js.ast.JsNameRef;
import com.google.gwt.dev.js.ast.JsNumberLiteral;
import com.google.gwt.dev.js.ast.JsPostfixOperation;
import com.google.gwt.dev.js.ast.JsPrefixOperation;
import com.google.gwt.dev.js.ast.JsRootScope;
import com.google.gwt.dev.js.ast.JsStatement;
import com.google.gwt.dev.js.ast.JsStringLiteral;
import com.google.gwt.dev.js.ast.JsUnaryOperator;
import com.google.gwt.dev.js.ast.JsValueLiteral;
import com.google.gwt.dev.js.ast.JsVisitor;
import com.google.gwt.user.client.rpc.IncompatibleRemoteServiceException;
import com.google.gwt.user.client.rpc.SerializationException;
import com.google.gwt.user.server.Base64Utils;

import java.io.StringReader;
import java.util.ArrayList;
import java.util.List;

/**
 * A version of ClientSerializationStreamReader which is optimized for performance in
 * devmode.
 * 
 * There is a super-src version of this class which is optimized for webmode performance.
 */
public final class ClientSerializationStreamReader extends
    AbstractSerializationStreamReader {

  /**
   * Decodes an RPC payload from a JS string. There is currently no design document that describes
   * the payload, instead you must infer it from reading ServerSerializationStreamWriter.
   * I'll briefly describe the payload here:
   * <p> 
   * The server sends down a string of JavaScript which is eval'ed by the webmode version of
   * ClientSerializationStreamReader into an array. The array contains primitive values,
   * followed by a nested array of strings, followed by a couple of header primitive values.  
   * For example,
   * <pre>
   * [ 1, 0, 3, -7, 13, [ "string one", "string two", "string three" ], 7, 0 ] 
   * </pre>
   * Long primitives are encoded as strings in the outer array, and strings in the string table
   * are referenced by index values in the outer array. 
   * <p> 
   * The payload is almost a JSON literal except for some nuances, like unicode and array concats.
   * We have a specialized devmode version to decode this payload, because the webmode version
   * requires multiple round-trips between devmode and the JSVM for every single element in the
   * payload. This can require several seconds to decode a single RPC payload. The RpcDecoder
   * operates by doing a limited JS parse on the payload within the devmode VM using Rhino,
   * avoiding the cross-process RPCs.  
   * <p> 
   */
  private static class RpcDecoder extends JsVisitor {

    private static final String JS_INFINITY_LITERAL = "Infinity";
    private static final String JS_NAN_LITERAL = "NaN";

    enum State {
      EXPECTING_PAYLOAD_BEGIN,
      EXPECTING_STRING_TABLE,
      IN_STRING_TABLE,
      EXPECTING_END
    }
    
    State state = State.EXPECTING_PAYLOAD_BEGIN;

    List<String> stringTable = new ArrayList<String>();
    List<JsValueLiteral> values = new ArrayList<JsValueLiteral>();
    
    boolean negative;

    @Override
    public void endVisit(JsArrayLiteral x, JsContext ctx) {
      if (state == State.IN_STRING_TABLE) {
        state = State.EXPECTING_END;
      }
    }

    public List<String> getStringTable() {
      return stringTable;
    }

    public List<JsValueLiteral> getValues() {
      return values;
    }
    
    @Override
    public boolean visit(JsArrayLiteral x, JsContext ctx) {
      switch (state) {
        case EXPECTING_PAYLOAD_BEGIN:
          state = State.EXPECTING_STRING_TABLE;
        return true;
        case EXPECTING_STRING_TABLE:
          state = State.IN_STRING_TABLE;
        return true;
        default:
        throw new RuntimeException("Unexpected array in RPC payload. The string table has " 
              + "already started.");                  
      }
    }
    
    @Override
    public boolean visit(JsBooleanLiteral x, JsContext ctx) {
      values.add(x);
      return true;
    }

    @Override
    public boolean visit(JsNameRef x, JsContext ctx) {
      String ident = x.getIdent();
      
      if (ident.equals(JS_NAN_LITERAL)) {
        values.add(new JsNumberLiteral(SourceOrigin.UNKNOWN, Double.NaN));
      } else if (ident.equals(JS_INFINITY_LITERAL)) {
        double val = negative ? Double.NEGATIVE_INFINITY : Double.POSITIVE_INFINITY;
        negative = false;
        values.add(new JsNumberLiteral(SourceOrigin.UNKNOWN, val));
      } else {
        throw new RuntimeException("Unexpected identifier: " + ident);
      }

      return true;
    }

    @Override
    public boolean visit(JsNumberLiteral x, JsContext ctx) {
      if (negative) {
        x = new JsNumberLiteral(x.getSourceInfo(), -x.getValue());
        negative = false;
      }
      values.add(x);
      return true;
    }

    @Override
    public boolean visit(JsPrefixOperation x, JsContext ctx) {
      if (x.getOperator().equals(JsUnaryOperator.NEG)) {
        negative = !negative;
        return true;
      }
      
      // Lots of prefix operators, but we only see negatives for literals
      throw new RuntimeException("Unexpected prefix operator: " + x.toSource()); 
    }

    @Override
    public boolean visit(JsPostfixOperation x, JsContext ctx) {
      throw new RuntimeException("Unexpected postfix operator: " + x.toSource());
    }

    @Override
    public boolean visit(JsStringLiteral x, JsContext ctx) {
      if (state == State.IN_STRING_TABLE) {
        stringTable.add(x.getValue());
      } else {
        values.add(x);
      }
      return true;
    }
  }

  private RpcDecoder decoder;
  private int index;
  private Serializer serializer;

  /**
   * The server breaks up large arrays in the RPC payload into smaller arrays using concat()
   * expressions. For example, [1, 2, 3, 4, 5, 6, 7] can be broken up into
   * [1, 2].concat([3, 4], [5, 6], [7,8])
   * <p> 
   * This visitor reverses that transform by reducing all concat invocations into a single array 
   * literal. 
   */
  private static class ArrayConcatEvaler extends JsModVisitor {

    @Override
    public boolean visit(JsInvocation invoke, JsContext ctx) {
      JsExpression expr = invoke.getQualifier();
      if (!(expr instanceof JsNameRef)) {
        return super.visit(invoke, ctx);
      }

      JsNameRef name = (JsNameRef) expr;
      if (!name.getIdent().equals("concat")) {
        return super.visit(invoke, ctx);
      }

      JsArrayLiteral headElements = (JsArrayLiteral) name.getQualifier();

      for (JsExpression ex : invoke.getArguments()) {
        JsArrayLiteral arg = (JsArrayLiteral) ex;
        headElements.getExpressions().addAll(arg.getExpressions());
      }

      ctx.replaceMe(headElements);
      return true;
    }
  }

  /**
   * The server splits up string literals into 64KB chunks using '+' operators. For example
   * ['chunk1chunk2'] is broken up into ['chunk1' + 'chunk2'].
   * <p>
   * This visitor reverses that transform by reducing such strings into a single string literal.
   */
  private static class StringConcatEvaler extends JsModVisitor {

    @Override public boolean visit(JsBinaryOperation x, JsContext ctx) {
      if (x.getOperator() != JsBinaryOperator.ADD) {
        return super.visit(x, ctx);
      }

      // Do a first pass to get the total string length to avoid dynamically resizing the buffer.
      int stringLength = getStringLength(x);
      if (stringLength >= 0) {
        StringBuilder builder = new StringBuilder(stringLength);
        if (expressionToString(x, builder)) {
          ctx.replaceMe(new JsStringLiteral(x.getSourceInfo(), builder.toString()));
        }
      }

      return true;
    }

    /**
     * Transforms an expression into a string. This will recurse into JsBinaryOperations of type
     * JsBinaryOperator.ADD, which may have other ADD operations or JsStringLiterals as arguments.
     *
     * @param expression the expression to evaluate
     * @param builder a builder that the string will be appended to
     * @return true if the expression represents a valid string, or false otherwise
     */
    private boolean expressionToString(JsExpression expression, StringBuilder builder) {
      if (expression instanceof JsStringLiteral) {
        builder.append(((JsStringLiteral) expression).getValue());
        return true;
      }

      if (expression instanceof JsBinaryOperation) {
        JsBinaryOperation operation = (JsBinaryOperation) expression;
        if (operation.getOperator() != JsBinaryOperator.ADD) {
          return false;
        }
        return expressionToString(operation.getArg1(), builder)
            && expressionToString(operation.getArg2(), builder);
      }

      return false;
    }

    /**
     * Gets the total string length of the given expression. This will recurse into
     * JsBinaryOperations of type JsBinaryOperator.ADD, which may have other ADD operations or
     * JsStringLiterals as arguments.
     *
     * @param expression the expression to evaluate
     * @return the total string length, or -1 if the given expression does not represent a valid
     *     string
     */
    private int getStringLength(JsExpression expression) {
      if (expression instanceof JsStringLiteral) {
        return ((JsStringLiteral) expression).getValue().length();
      }

      if (expression instanceof JsBinaryOperation) {
        JsBinaryOperation operation = (JsBinaryOperation) expression;
        if (operation.getOperator() != JsBinaryOperator.ADD) {
          return -1;
        }

        int arg1Length = getStringLength(operation.getArg1());
        int arg2Length = getStringLength(operation.getArg2());

        return (arg1Length >= 0 && arg2Length >= 0) ? (arg1Length + arg2Length) : -1;
      }

      return -1;
    }
  }

  public ClientSerializationStreamReader(Serializer serializer) {
    this.serializer = serializer;
  }

  @Override
  public void prepareToRead(String encoded) throws SerializationException {
    try {
      List<JsStatement> stmts = JsParser.parse(SourceOrigin.UNKNOWN, JsRootScope.INSTANCE,
          new StringReader(encoded));
      ArrayConcatEvaler arrayConcatEvaler = new ArrayConcatEvaler();
      arrayConcatEvaler.acceptList(stmts);
      StringConcatEvaler stringConcatEvaler = new StringConcatEvaler();
      stringConcatEvaler.acceptList(stmts);
      decoder = new RpcDecoder();
      decoder.acceptList(stmts);
    } catch (Exception e) {
      throw new SerializationException("Failed to parse RPC payload", e);
    }

    index = decoder.getValues().size();
    super.prepareToRead(encoded);

    if (getVersion() < SERIALIZATION_STREAM_MIN_VERSION
        || getVersion() > SERIALIZATION_STREAM_MAX_VERSION) {
      throw new IncompatibleRemoteServiceException("Got version " + getVersion()
          + ", expected version between " + SERIALIZATION_STREAM_MIN_VERSION + " and "
          + SERIALIZATION_STREAM_MAX_VERSION);
    }

    if (!areFlagsValid()) {
      throw new IncompatibleRemoteServiceException("Got an unknown flag from "
          + "server: " + getFlags());
    }
  }

  @Override
  public boolean readBoolean() {
    JsValueLiteral literal = decoder.getValues().get(--index);
    return literal.isBooleanTrue();
  }

  @Override
  public byte readByte() {    
    JsNumberLiteral literal = (JsNumberLiteral) decoder.getValues().get(--index);
    return (byte) literal.getValue();
  }
  
  @Override
  public char readChar() {    
    JsNumberLiteral literal = (JsNumberLiteral) decoder.getValues().get(--index);
    return (char) literal.getValue();
  }
  
  @Override
  public double readDouble() {    
    JsValueLiteral valueLiteral = decoder.getValues().get(--index);
    if (valueLiteral instanceof JsNumberLiteral) {
      JsNumberLiteral literal = (JsNumberLiteral) valueLiteral;
      return literal.getValue();
    } else if (valueLiteral instanceof JsStringLiteral) {
      JsStringLiteral literal = (JsStringLiteral) valueLiteral;
      return Double.parseDouble(literal.getValue());
    } else {
      throw new RuntimeException("Can't read double from " + valueLiteral.getKind() + " literal");
    }
  }
  
  @Override
  public float readFloat() {    
    return (float) readDouble();
  }
  
  @Override
  public int readInt() {    
    JsNumberLiteral literal = (JsNumberLiteral) decoder.getValues().get(--index);
    return (int) literal.getValue();
  }
  
  @Override
  public long readLong() {    
    return Base64Utils.longFromBase64(
        ((JsStringLiteral) decoder.getValues().get(--index)).getValue());
  }
  
  @Override
  public short readShort() {    
    JsNumberLiteral literal = (JsNumberLiteral) decoder.getValues().get(--index);
    return (short) literal.getValue();
  }
  
  @Override
  public String readString() {
    return getString(readInt());
  }

  @Override
  protected Object deserialize(String typeSignature)
      throws SerializationException {
    int id = reserveDecodedObjectIndex();
    Object instance = serializer.instantiate(this, typeSignature);
    rememberDecodedObject(id, instance);
    serializer.deserialize(this, instance, typeSignature);
    return instance;
  }

  @Override
  protected String getString(int index) {
    // index is 1-based
    return index > 0 ? decoder.getStringTable().get(index - 1) : null;
  }  
}