MyBatis印象阅读之NodeHandler和SqlNode解

在上一章中我们讲到了choose (when, otherwise)标签的源码分析，今天我们继续来往下讲。

1 NodeHandler与SqlNode相关源码分析

先来分析：trim (where, set)源码

我们先来看TrimHandler：

private class TrimHandler implements NodeHandler {
    public TrimHandler() {
      // Prevent Synthetic Access
    }

    @Override
    public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
      MixedSqlNode mixedSqlNode = parseDynamicTags(nodeToHandle);
      String prefix = nodeToHandle.getStringAttribute("prefix");
      String prefixOverrides = nodeToHandle.getStringAttribute("prefixOverrides");
      String suffix = nodeToHandle.getStringAttribute("suffix");
      String suffixOverrides = nodeToHandle.getStringAttribute("suffixOverrides");
      TrimSqlNode trim = new TrimSqlNode(configuration, mixedSqlNode, prefix, prefixOverrides, suffix, suffixOverrides);
      targetContents.add(trim);
    }
  }

这里没有什么难点，就是获取参数并创建TrimSqlNode节点。
我们重点来看，由于这个类比较长 我们逐步来分析，先来看属性和构造方法：

public class TrimSqlNode implements SqlNode {

  private final SqlNode contents;
  private final String prefix;
  private final String suffix;
  private final List<String> prefixesToOverride;
  private final List<String> suffixesToOverride;
  private final Configuration configuration;

  public TrimSqlNode(Configuration configuration, SqlNode contents, String prefix, String prefixesToOverride, String suffix, String suffixesToOverride) {
    this(configuration, contents, prefix, parseOverrides(prefixesToOverride), suffix, parseOverrides(suffixesToOverride));
  }

  protected TrimSqlNode(Configuration configuration, SqlNode contents, String prefix, List<String> prefixesToOverride, String suffix, List<String> suffixesToOverride) {
    this.contents = contents;
    this.prefix = prefix;
    this.prefixesToOverride = prefixesToOverride;
    this.suffix = suffix;
    this.suffixesToOverride = suffixesToOverride;
    this.configuration = configuration;
  }
}

其中有个方法是:

  private static List<String> parseOverrides(String overrides) {
    if (overrides != null) {
      final StringTokenizer parser = new StringTokenizer(overrides, "|", false);
      final List<String> list = new ArrayList<>(parser.countTokens());
      while (parser.hasMoreTokens()) {
        list.add(parser.nextToken().toUpperCase(Locale.ENGLISH));
      }
      return list;
    }
    return Collections.emptyList();
  }

这块代码逻辑还是很简单的，把overrides通过|分割形成一个list。

之后我们再来看它的主要方法：

  @Override
  public boolean apply(DynamicContext context) {
    FilteredDynamicContext filteredDynamicContext = new FilteredDynamicContext(context);
    boolean result = contents.apply(filteredDynamicContext);
    filteredDynamicContext.applyAll();
    return result;
  }

这里的FilteredDynamicContext是内部类：

  private class FilteredDynamicContext extends DynamicContext {
    private DynamicContext delegate;
    private boolean prefixApplied;
    private boolean suffixApplied;
    private StringBuilder sqlBuffer;

    public FilteredDynamicContext(DynamicContext delegate) {
      super(configuration, null);
      this.delegate = delegate;
      this.prefixApplied = false;
      this.suffixApplied = false;
      this.sqlBuffer = new StringBuilder();
    }

    public void applyAll() {
      sqlBuffer = new StringBuilder(sqlBuffer.toString().trim());
      String trimmedUppercaseSql = sqlBuffer.toString().toUpperCase(Locale.ENGLISH);
      if (trimmedUppercaseSql.length() > 0) {
        applyPrefix(sqlBuffer, trimmedUppercaseSql);
        applySuffix(sqlBuffer, trimmedUppercaseSql);
      }
      delegate.appendSql(sqlBuffer.toString());
    }

    @Override
    public Map<String, Object> getBindings() {
      return delegate.getBindings();
    }

    @Override
    public void bind(String name, Object value) {
      delegate.bind(name, value);
    }

    @Override
    public int getUniqueNumber() {
      return delegate.getUniqueNumber();
    }

    @Override
    public void appendSql(String sql) {
      sqlBuffer.append(sql);
    }

    @Override
    public String getSql() {
      return delegate.getSql();
    }

    private void applyPrefix(StringBuilder sql, String trimmedUppercaseSql) {
      if (!prefixApplied) {
        prefixApplied = true;
        if (prefixesToOverride != null) {
          for (String toRemove : prefixesToOverride) {
            if (trimmedUppercaseSql.startsWith(toRemove)) {
              sql.delete(0, toRemove.trim().length());
              break;
            }
          }
        }
        if (prefix != null) {
          sql.insert(0, " ");
          sql.insert(0, prefix);
        }
      }
    }

    private void applySuffix(StringBuilder sql, String trimmedUppercaseSql) {
      if (!suffixApplied) {
        suffixApplied = true;
        if (suffixesToOverride != null) {
          for (String toRemove : suffixesToOverride) {
            if (trimmedUppercaseSql.endsWith(toRemove) || trimmedUppercaseSql.endsWith(toRemove.trim())) {
              int start = sql.length() - toRemove.trim().length();
              int end = sql.length();
              sql.delete(start, end);
              break;
            }
          }
        }
        if (suffix != null) {
          sql.append(" ");
          sql.append(suffix);
        }
      }
    }

  }

这个类使用了代理模式来进行包装，实际逻辑大家自行看下，描述起来可能比较费劲。结合我们实际使用trim标签的应用来看会更好。

分析完了trimHander，我们再来看whereHander：

 private class WhereHandler implements NodeHandler {
    public WhereHandler() {
      // Prevent Synthetic Access
    }

    @Override
    public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
      MixedSqlNode mixedSqlNode = parseDynamicTags(nodeToHandle);
      WhereSqlNode where = new WhereSqlNode(configuration, mixedSqlNode);
      targetContents.add(where);
    }
  }

public class WhereSqlNode extends TrimSqlNode {

  private static List<String> prefixList = Arrays.asList("AND ","OR ","AND\n", "OR\n", "AND\r", "OR\r", "AND\t", "OR\t");

  public WhereSqlNode(Configuration configuration, SqlNode contents) {
    super(configuration, contents, "WHERE", prefixList, null, null);
  }

}

看到这是不是松了口气，继承了TrimSqlNode说明我们不需要再去了解另外一个类了，逻辑跟trim一样。

下一个setHandler：

 private class SetHandler implements NodeHandler {
    public SetHandler() {
      // Prevent Synthetic Access
    }

    @Override
    public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
      MixedSqlNode mixedSqlNode = parseDynamicTags(nodeToHandle);
      SetSqlNode set = new SetSqlNode(configuration, mixedSqlNode);
      targetContents.add(set);
    }
  }

对应的sqlNode：

public class SetSqlNode extends TrimSqlNode {

  private static final List<String> COMMA = Collections.singletonList(",");

  public SetSqlNode(Configuration configuration,SqlNode contents) {
    super(configuration, contents, "SET", COMMA, null, COMMA);
  }

}

也一样跟trimHandler相关，我们也不做赘述。

最后一个是foreach标签:

 private class ForEachHandler implements NodeHandler {
    public ForEachHandler() {
      // Prevent Synthetic Access
    }

    @Override
    public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
      MixedSqlNode mixedSqlNode = parseDynamicTags(nodeToHandle);
      String collection = nodeToHandle.getStringAttribute("collection");
      String item = nodeToHandle.getStringAttribute("item");
      String index = nodeToHandle.getStringAttribute("index");
      String open = nodeToHandle.getStringAttribute("open");
      String close = nodeToHandle.getStringAttribute("close");
      String separator = nodeToHandle.getStringAttribute("separator");
      ForEachSqlNode forEachSqlNode = new ForEachSqlNode(configuration, mixedSqlNode, collection, index, item, open, close, separator);
      targetContents.add(forEachSqlNode);
    }
  }

这里不难理解，主要是关键ForEachSqlNode，我们直接来看：

public class ForEachSqlNode implements SqlNode {
  public static final String ITEM_PREFIX = "__frch_";

  private final ExpressionEvaluator evaluator;
  private final String collectionExpression;
  private final SqlNode contents;
  private final String open;
  private final String close;
  private final String separator;
  private final String item;
  private final String index;
  private final Configuration configuration;

  public ForEachSqlNode(Configuration configuration, SqlNode contents, String collectionExpression, String index, String item, String open, String close, String separator) {
    this.evaluator = new ExpressionEvaluator();
    this.collectionExpression = collectionExpression;
    this.contents = contents;
    this.open = open;
    this.close = close;
    this.separator = separator;
    this.index = index;
    this.item = item;
    this.configuration = configuration;
  }

  @Override
  public boolean apply(DynamicContext context) {
    Map<String, Object> bindings = context.getBindings();
    final Iterable<?> iterable = evaluator.evaluateIterable(collectionExpression, bindings);
    if (!iterable.iterator().hasNext()) {
      return true;
    }
    boolean first = true;
    applyOpen(context);
    int i = 0;
    for (Object o : iterable) {
      DynamicContext oldContext = context;
      if (first || separator == null) {
        context = new PrefixedContext(context, "");
      } else {
        context = new PrefixedContext(context, separator);
      }
      int uniqueNumber = context.getUniqueNumber();
      // Issue #709
      if (o instanceof Map.Entry) {
        @SuppressWarnings("unchecked")
        Map.Entry<Object, Object> mapEntry = (Map.Entry<Object, Object>) o;
        applyIndex(context, mapEntry.getKey(), uniqueNumber);
        applyItem(context, mapEntry.getValue(), uniqueNumber);
      } else {
        applyIndex(context, i, uniqueNumber);
        applyItem(context, o, uniqueNumber);
      }
      contents.apply(new FilteredDynamicContext(configuration, context, index, item, uniqueNumber));
      if (first) {
        first = !((PrefixedContext) context).isPrefixApplied();
      }
      context = oldContext;
      i++;
    }
    applyClose(context);
    context.getBindings().remove(item);
    context.getBindings().remove(index);
    return true;
  }

  private void applyIndex(DynamicContext context, Object o, int i) {
    if (index != null) {
      context.bind(index, o);
      context.bind(itemizeItem(index, i), o);
    }
  }

  private void applyItem(DynamicContext context, Object o, int i) {
    if (item != null) {
      context.bind(item, o);
      context.bind(itemizeItem(item, i), o);
    }
  }

  private void applyOpen(DynamicContext context) {
    if (open != null) {
      context.appendSql(open);
    }
  }

  private void applyClose(DynamicContext context) {
    if (close != null) {
      context.appendSql(close);
    }
  }

  private static String itemizeItem(String item, int i) {
    return ITEM_PREFIX + item + "_" + i;
  }

  private static class FilteredDynamicContext extends DynamicContext {
    private final DynamicContext delegate;
    private final int index;
    private final String itemIndex;
    private final String item;

    public FilteredDynamicContext(Configuration configuration,DynamicContext delegate, String itemIndex, String item, int i) {
      super(configuration, null);
      this.delegate = delegate;
      this.index = i;
      this.itemIndex = itemIndex;
      this.item = item;
    }

    @Override
    public Map<String, Object> getBindings() {
      return delegate.getBindings();
    }

    @Override
    public void bind(String name, Object value) {
      delegate.bind(name, value);
    }

    @Override
    public String getSql() {
      return delegate.getSql();
    }

    @Override
    public void appendSql(String sql) {
      GenericTokenParser parser = new GenericTokenParser("#{", "}", content -> {
        String newContent = content.replaceFirst("^\\s*" + item + "(?![^.,:\\s])", itemizeItem(item, index));
        if (itemIndex != null && newContent.equals(content)) {
          newContent = content.replaceFirst("^\\s*" + itemIndex + "(?![^.,:\\s])", itemizeItem(itemIndex, index));
        }
        return "#{" + newContent + "}";
      });

      delegate.appendSql(parser.parse(sql));
    }

    @Override
    public int getUniqueNumber() {
      return delegate.getUniqueNumber();
    }

  }


  private class PrefixedContext extends DynamicContext {
    private final DynamicContext delegate;
    private final String prefix;
    private boolean prefixApplied;

    public PrefixedContext(DynamicContext delegate, String prefix) {
      super(configuration, null);
      this.delegate = delegate;
      this.prefix = prefix;
      this.prefixApplied = false;
    }

    public boolean isPrefixApplied() {
      return prefixApplied;
    }

    @Override
    public Map<String, Object> getBindings() {
      return delegate.getBindings();
    }

    @Override
    public void bind(String name, Object value) {
      delegate.bind(name, value);
    }

    @Override
    public void appendSql(String sql) {
      if (!prefixApplied && sql != null && sql.trim().length() > 0) {
        delegate.appendSql(prefix);
        prefixApplied = true;
      }
      delegate.appendSql(sql);
    }

    @Override
    public String getSql() {
      return delegate.getSql();
    }

    @Override
    public int getUniqueNumber() {
      return delegate.getUniqueNumber();
    }
  }

}

2. 今日总结

关于解析这块讲的有点云里雾里的，主要这里逻辑比较复杂，比较难讲清，建议结合测试Demo来进行调试分析。