源码中一些好的代码写法

1  Map相关

摘自 Spring中加载 META-INF/spring.factories 下的类名，Map的巧初始化：

Map<String, List<String>> result = cache.get(classLoader);
// 缓存中有就直接返回
if (result != null) {
    return result;
}
// 初始化Map
result = new HashMap<>();
// value是个集合，初始化value并放数据
result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>()).add(factoryImplementationName.trim());
// 也可以这么写
result.putIfAbsent(factoryTypeName, new ArrayList<>()).add(factoryImplementationName.trim());
// 对value去重并转换为可读集合
result.replaceAll((factoryType, implementations) -> implementations.stream().distinct().collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));

2  字符串相关

摘自 Spring中加载 META-INF/spring.factories 下的类名，逗号分隔：

// 逗号分隔出来每个全类名
String[] factoryImplementationNames = StringUtils.commaDelimitedListToStringArray((String) entry.getValue());

3  对象相关

对象比较
ObjectUtils.nullSafeEquals(this.sourceType, otherKey.sourceType));
public static boolean nullSafeEquals(@Nullable Object o1, @Nullable Object o2) {
    if (o1 == o2) {
        return true;
    }
    if (o1 == null || o2 == null) {
        return false;
    }
    if (o1.equals(o2)) {
        return true;
    }
    if (o1.getClass().isArray() && o2.getClass().isArray()) {
        return arrayEquals(o1, o2);
    }
    return false;
}

4  写法相关

4.1  标志移除

public <T> void addMapper(Class<T> type) {
  if (type.isInterface()) {
    if (hasMapper(type)) {
      throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
    }
    boolean loadCompleted = false;
    try {
      /*
       * 将 type 和 MapperProxyFactory 进行绑定，
       * MapperProxyFactory 来为 mapper 接口生成代理类
       */
      knownMappers.put(type, new MapperProxyFactory<>(type));
      // It's important that the type is added before the parser is run
      // otherwise the binding may automatically be attempted by the
      // mapper parser. If the type is already known, it won't try.
      // 创建注解解析器。在 MyBatis 中，有 XML 和 注解两种配置方式可选
      MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
      // 解析注解中的信息
      parser.parse();
      loadCompleted = true;
    } finally {
      if (!loadCompleted) {
        knownMappers.remove(type);
      }
    }
  }
}

private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();
public <T> void addMapper(Class<T> type) {
  if (type.isInterface()) {
    if (hasMapper(type)) {
      throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
    }
    boolean loadCompleted = false;
    try {
      // 业务操作
      loadCompleted = true;
    } finally {
      if (!loadCompleted) {
        knownMappers.remove(type);
      }
    }
  }
}