阅读开源框架总结Java类的定义

Java的类是自定义的引用类型,是对职责相关的行为与数据的一种封装,用以表现一种业务领域或者技术领域的概念。在不同的场景,类包含的成员可能有所不同,大体可以分为如下五类:

  • 数据类:可以视为是持有数据的容器,类的成员只包含了字段,以及与字段有关的get/set方法
  • 实体类:既包含了体现状态的字段,又包含了操作这些状态的方法
  • 服务类:只有方法(行为)没有字段(状态),可以理解为提供内聚职责的服务
  • 函数类:如果定义的公开方法只有唯一一个,可以理解为它封装的其实是一个函数,通常用匿名类或者Lambda表示
  • 工具类:只包含一系列静态方法,通常不支持对该类型的实例化

数据类

在Presto框架中定义的ClientSession可以认为是这样一种数据类。除了构造函数外,它只定义了字段与对应的get()方法(实际上,在框架的源代码中,在ClientSession类中还定义了一系列静态工厂方法,但本质上说,ClientSession还是一个数据类),用以持有客户端Session所必须的数据:

public class ClientSession
{
private final URI server;
private final String user;
private final String source;
private final String clientInfo;
private final String catalog;
private final String schema;
private final TimeZoneKey timeZone;
private final Locale locale;
private final Map<String, String> properties;
private final Map<String, String> preparedStatements;
private final String transactionId;
private final boolean debug;
private final Duration clientRequestTimeout;

public ClientSession(
URI server,
String user,
String source,
String clientInfo,
String catalog,
String schema,
String timeZoneId,
Locale locale,
Map<String, String> properties,
String transactionId,
boolean debug,
Duration clientRequestTimeout)
{
this(server, user, source, clientInfo, catalog, schema, timeZoneId, locale, properties, emptyMap(), transactionId, debug, clientRequestTimeout);
}

public ClientSession(
URI server,
String user,
String source,
String clientInfo,
String catalog,
String schema,
String timeZoneId,
Locale locale,
Map<String, String> properties,
Map<String, String> preparedStatements,
String transactionId,
boolean debug,
Duration clientRequestTimeout)
{
this.server = requireNonNull(server, "server is null");
this.user = user;
this.source = source;
this.clientInfo = clientInfo;
this.catalog = catalog;
this.schema = schema;
this.locale = locale;
this.timeZone = TimeZoneKey.getTimeZoneKey(timeZoneId);
this.transactionId = transactionId;
this.debug = debug;
this.properties = ImmutableMap.copyOf(requireNonNull(properties, "properties is null"));
this.preparedStatements = ImmutableMap.copyOf(requireNonNull(preparedStatements, "preparedStatements is null"));
this.clientRequestTimeout = clientRequestTimeout;

// verify the properties are valid
CharsetEncoder charsetEncoder = US_ASCII.newEncoder();
for (Entry<String, String> entry : properties.entrySet()) {
checkArgument(!entry.getKey().isEmpty(), "Session property name is empty");
checkArgument(entry.getKey().indexOf('=') < 0, "Session property name must not contain '=': %s", entry.getKey());
checkArgument(charsetEncoder.canEncode(entry.getKey()), "Session property name is not US_ASCII: %s", entry.getKey());
checkArgument(charsetEncoder.canEncode(entry.getValue()), "Session property value is not US_ASCII: %s", entry.getValue());
}
}

public URI getServer()
{
return server;
}

public String getUser()
{
return user;
}

public String getSource()
{
return source;
}

public String getClientInfo()
{
return clientInfo;
}

public String getCatalog()
{
return catalog;
}

public String getSchema()
{
return schema;
}

public TimeZoneKey getTimeZone()
{
return timeZone;
}

public Locale getLocale()
{
return locale;
}

public Map<String, String> getProperties()
{
return properties;
}

public Map<String, String> getPreparedStatements()
{
return preparedStatements;
}

public String getTransactionId()
{
return transactionId;
}

public boolean isDebug()
{
return debug;
}

public Duration getClientRequestTimeout()
{
return clientRequestTimeout;
}

@Override
public String toString()
{
return toStringHelper(this)
.add("server", server)
.add("user", user)
.add("clientInfo", clientInfo)
.add("catalog", catalog)
.add("schema", schema)
.add("timeZone", timeZone)
.add("locale", locale)
.add("properties", properties)
.add("transactionId", transactionId)
.add("debug", debug)
.toString();
}
}

这样包含数据或状态的对象通常会作为参数在方法调用之间传递,体现了诸如配置、视图模型、服务传输数据、协议数据等概念。除此之外,我们应尽量避免定义这样的对象去体现某种业务概念,因为基于“信息专家”模式,好的面向对象设计应该是将数据与操作这些数据的行为封装在一起。

实体类

这是最为常见的一种类定义,也是符合面向对象设计原则的,前提是定义的类必须是高内聚的,原则上应该满足单一职责原则。例如JDK定义的Vector展现了一种数据结构,因而它持有的字段与方法应该仅仅与队列操作与状态有关:

public class Vector<E>
extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{

protected Object[] elementData;
protected int elementCount;
protected int capacityIncrement;

public Vector(int initialCapacity, int capacityIncrement) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
this.capacityIncrement = capacityIncrement;
}

public Vector(int initialCapacity) {
this(initialCapacity, 0);
}

public synchronized void setSize(int newSize) {
modCount++;
if (newSize > elementCount) {
ensureCapacityHelper(newSize);
} else {
for (int i = newSize ; i < elementCount ; i++) {
elementData[i] = null;
}
}
elementCount = newSize;
}

public synchronized int size() {
return elementCount;
}

public synchronized boolean isEmpty() {
return elementCount == 0;
}

public boolean contains(Object o) {
return indexOf(o, 0) >= 0;
}

public synchronized E firstElement() {
if (elementCount == 0) {
throw new NoSuchElementException();
}
return elementData(0);
}

public synchronized void insertElementAt(E obj, int index) {
modCount++;
if (index > elementCount) {
throw new ArrayIndexOutOfBoundsException(index
+ " > " + elementCount);
}
ensureCapacityHelper(elementCount + 1);
System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);
elementData[index] = obj;
elementCount++;
}

public synchronized void addElement(E obj) {
modCount++;
ensureCapacityHelper(elementCount + 1);
elementData[elementCount++] = obj;
}

public synchronized boolean removeElement(Object obj) {
modCount++;
int i = indexOf(obj);
if (i >= 0) {
removeElementAt(i);
return true;
}
return false;
}

public synchronized void removeAllElements() {
modCount++;
// Let gc do its work
for (int i = 0; i < elementCount; i++)
elementData[i] = null;

elementCount = 0;
}
}

如下类的定义则体现了一种业务概念,方法changePriceTo()实际上表现的是一种业务规则,而它要操作的数据就是Product类自身持有的字段sellingPrice

public class Product extends Entity<Identity> {
private final List<Option> options;
private Price sellingPrice;
private Price retailPrice;

public Product(Identity id, Price sellingPrice, Price retailPrice) {
super(id);
this.sellingPrice = sellingPrice;
if (!sellingPriceMatches(retailPrice) {
throw new PricesNotInTheSameCurrencyException("Selling and retail price must be in the same currency");
}
this.retailPrice = retailPrice;
options = new List<Option>();
}

public void changePriceTo(Price newPrice) {
if (!sellingPriceMatches(newPrice)) {
throw new PricesNotInTheSameCurrencyException("You cannot change the price of this product to a different currency");
}
sellingPrice = newPrice;
}

public Price savings() {
Price savings = retailPrice.minus(sellingPrice);
if (savings.isGreaterThanZero())
return savings;
else
return new Price(0m, sellingPrice.currency);
}

private bool sellingPriceMatches(Price retailPrice) {
return sellingPrice.sameCurrency(retailPrice);
}

public void add(Option option) {
if (!this.contains(option))
options.Add(option);
else
throw new ProductOptionAddedNotUniqueException(string.Format("This product already has the option {0}", option.ToString()));
}

public bool contains(Option option) {
return options.Contains(option);
}
}

服务类

只有方法没有状态的类定义是对行为的封装,行为的实现要么是通过操作内部封装的不可变私有数据,要么是通过操作传入的参数对象实现对状态的修改。由于参数传入的状态与服务类自身没有任何关系,因此这样的类通常也被视为无状态的类。以下代码是针对升级激活包的验证服务:

public class PreActivePackageValidator {
public long validatePreActivePackage(ActiveManifest activeManifest) {
validateSamePackageType(activeManifest);
validateNoTempPackage(activeManifest);
validateNoPackageRunning(activeManifest);
validateAllPackagesBeenDownloaded(activeManifest);
validateNoFatherPackageBakStatus(activeManifest);
validatePackageNum(activeManifest);
}
private void validateSamePackageType(ActiveManifest activeManifest) {
int packakeType = activeManifest.getPackageType();
for (UpagrdePackage pkg : activeManifest.getPackages()) {
if (packageType != pkg.getPackageType()) {
throw new PackagePreActiveException("pre active exist different type package");
}
}
}
}

服务类还可以操作外部资源,例如读取文件、访问数据库、与第三方服务通信等。例如airlift框架定义的ConfigurationLoader类,就提供加载配置文件内容的服务:

public class ConfigurationLoader {
public Map<String, String> loadProperties()
throws IOException {
Map<String, String> result = new TreeMap<>();
String configFile = System.getProperty("config");
if (configFile != null) {
result.putAll(loadPropertiesFrom(configFile));
}

result.putAll(getSystemProperties());

return ImmutableSortedMap.copyOf(result);
}

public Map<String, String> loadPropertiesFrom(String path)
throws IOException {
Properties properties = new Properties();
try (Reader reader = new FileReader(new File(path))) {
properties.load(reader);
}

return fromProperties(properties);
}

public Map<String, String> getSystemProperties() {
return fromProperties(System.getProperties());
}
}

函数类

可以将函数类理解为设计一个类,它仅仅实现了一个接口,且该接口只定义一个方法。使用时,我们会基于依赖倒置原则(DIP)从接口的角度使用这个类。为了重用的目的,这个类可以单独被定义,也可能体现为匿名类,或者Java 8中的Lambda表达式。

单独类形式

例如,在Presto中定义了PagesIndexComparator接口,提供了比较方法以用于支持对页面索引的排序。接口的定义为:

public interface PagesIndexComparator {
int compareTo(PagesIndex pagesIndex, int leftPosition, int rightPosition);
}

Presto定义了该接口的实现类SimplePagesIndexComparator,该类就是一个函数类:

public class SimplePagesIndexComparator
implements PagesIndexComparator {
private final List<Integer> sortChannels;
private final List<SortOrder> sortOrders;
private final List<Type> sortTypes;

public SimplePagesIndexComparator(List<Type> sortTypes, List<Integer> sortChannels, List<SortOrder> sortOrders) {
this.sortTypes = ImmutableList.copyOf(requireNonNull(sortTypes, "sortTypes is null"));
this.sortChannels = ImmutableList.copyOf(requireNonNull(sortChannels, "sortChannels is null"));
this.sortOrders = ImmutableList.copyOf(requireNonNull(sortOrders, "sortOrders is null"));
}

@Override
public int compareTo(PagesIndex pagesIndex, int leftPosition, int rightPosition) {
long leftPageAddress = pagesIndex.getValueAddresses().getLong(leftPosition);
int leftBlockIndex = decodeSliceIndex(leftPageAddress);
int leftBlockPosition = decodePosition(leftPageAddress);

long rightPageAddress = pagesIndex.getValueAddresses().getLong(rightPosition);
int rightBlockIndex = decodeSliceIndex(rightPageAddress);
int rightBlockPosition = decodePosition(rightPageAddress);

for (int i = 0; i < sortChannels.size(); i++) {
int sortChannel = sortChannels.get(i);
Block leftBlock = pagesIndex.getChannel(sortChannel).get(leftBlockIndex);
Block rightBlock = pagesIndex.getChannel(sortChannel).get(rightBlockIndex);

SortOrder sortOrder = sortOrders.get(i);
int compare = sortOrder.compareBlockValue(sortTypes.get(i), leftBlock, leftBlockPosition, rightBlock, rightBlockPosition);
if (compare != 0) {
return compare;
}
}
return 0;
}
}

我们看到SimplePagesIndexComparator类的逻辑相对比较复杂,构造函数也需要传入三个参数:List<Type> sortTypesList<Integer> sortChannelsList<SortOrder> sortOrders。虽然从接口的角度看,其实代表的是compare的语义,但由于逻辑复杂,而且需要传入三个对象帮助对PagesIndex进行比较,因而不可能实现为匿名类或者Lambda表达式。在Presto中,对它的使用为:

public class PagesIndexOrdering {
private final PagesIndexComparator comparator;

public PagesIndexOrdering(PagesIndexComparator comparator) {
this.comparator = requireNonNull(comparator, "comparator is null");
}

public PagesIndexComparator getComparator() {
return comparator;
}

/**
* Returns the index of the median of the three positions.
*/
private int median3(PagesIndex pagesIndex, int a, int b, int c) {
int ab = comparator.compareTo(pagesIndex, a, b);
int ac = comparator.compareTo(pagesIndex, a, c);
int bc = comparator.compareTo(pagesIndex, b, c);
return (ab < 0 ?
(bc < 0 ? b : ac < 0 ? c : a) :
(bc > 0 ? b : ac > 0 ? c : a));
}
}
匿名类形式

同样在该框架下定义的IntComparator接口,它的实现就完全不同了。首先是该接口的定义:

public interface IntComparator {
/** Compares the given primitive types.
*
* @see java.util.Comparator
* @return A positive integer, zero, or a negative integer if the first
* argument is greater than, equal to, or smaller than, respectively, the
* second one.
*/
int compare(int k1, int k2);
}

在针对整型数据提供排序功能时,用到了IntComparator接口:

public final class IntBigArray {
public void sort(int from, int to, IntComparator comparator) {
IntBigArrays.quickSort(array, from, to, comparator);
}
}

但由于提供整型数据的比较逻辑相对简单,在Presto中并没有定义显式的函数类,而是使用了Lambda表达式:

groupIds.sort(0, groupByHash.getGroupCount(), (leftGroupId, rightGroupId) ->
Long.compare(groupByHash.getRawHash(leftGroupId), groupByHash.getRawHash(rightGroupId)));

这里的Lambda表达式其实也可以理解为是一个函数类。

函数重用形式

还有一种特殊的函数类,它的定义形式与后面介绍的工具类非常相似,同样是定义了一组静态方法,但它的目的不是提供工具或辅助功能,而是将其视为函数成为被重用的单元。这时,需要用到Java 8提供的方法引用(method reference)语法。例如我们要对List<Apple>集合进行过滤,过滤条件分别为颜色与重量,这时可以在Apple类中定义两个静态方法:

public class Apple {
public static boolean isGreenApple(Apple apple) {
return "green".equals(apple.getColor());
}

public static boolean isHeavyApple(Apple apple) {
return apple.getWeight() > 150;
}
}

这两个方法实际上满足函数接口Predicate<Apple>的定义,因此可以在filter方法中传入这两个方法的引用:

public List<Apple> filter(Predicate<Apple> predicate) {
ArrayList<Apple> result = new ArrayList<>();
for (Apple apple : apples) {
if (predicate.test(apple)) {
result.add(apple);
}
}
return result;
}

public List<Apple> filterGreenApples() {
return filter(Apple::isGreenApple);
}

public List<Apple> filterHeavyApples() {
return filter(Apple::isHeavyApple);
}

此时Apple类可以认为是一个函数类,但准确地说法是一系列可以被重用的函数的容器。与工具类不同的是,这些函数并不是被直接调用,本质上讲,其实是作为“高阶函数”被传递给其他方法而被重用。虽然说实例方法也可以采用这种方式而被重用,但静态方法的调用会更加简单。

工具类

在许多项目或开源项目中,随处可见工具类的身影。无需实例化的特性使得我们使用工具类的方式时变得非常的便利,也不需要考虑状态的维护。然而越是方便,我们越是要警惕工具类的陷阱——设计出臃肿庞大无所不能的上帝工具类。工具类仍然要遵循高内聚的原则,只有强相关的职责才能放到同一个工具类中。

在定义工具类时,通常有三类命名范式:

  • 名词复数形式:工具类其实就是一系列工具方法的容器,当我们要针对某种类型(或对象)提供工具方法时,可以直接将工具类命名为该类型的复数形式,例如操作Collection的工具类可以命名为Collections,操作Object的工具类可以命名为Objects,而与前置条件有关的工具类则被命名为Preconditions
  • 以Util为后缀:这体现了工具(Utility)的语义,当我们在类名中看到Util后缀时,就可以直观地了解到这是一个工具类。例如ArrayUtil类是针对数组的工具类,DatabaseUtil是针对数据库操作的工具类,UuidUtil是针对Uuid的工具类。
  • 以Helper为后缀:这种命名相对较少,但许多框架也采用这种命名方式来体现“辅助类”的含义。例如在Druid框架中,就定义了JobHelperGroupByQueryHelper等辅助类。

工具类是无需实例化的,因此在定义工具类时,尽可能将其声明为final类,并为其定义私有的构造函数。例如Guava框架提供的Preconditions工具类:

public final class Preconditions {
private Preconditions() {
}

public static void checkArgument(boolean expression) {
if(!expression) {
throw new IllegalArgumentException();
}
}

//other util methods
}
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