1 概述
在看Netty服务端启动相关内容时,在配置channel的option和attr时(可参考笔者文章Netty源码-服务端启动过程),看到Netty使用常量池ConstantPool
进一步较少了小粒度对象的创建,Netty在提供性能真的是做了许多的努力。
Netty常量池ConstantPool
的实现比较简单,本文也就简单介绍一下。
2 常量池ConstantPool
Netty在ServerBootstrap
设置通道选项ChannelOption
或者属性AttributeKey
时都使用了常量池ConstantPool
,在AttributeKey
中定义了ConstantPool
子类如下:
//AttributeKey
private static final ConstantPool<AttributeKey<Object>> pool = new ConstantPool<AttributeKey<Object>>() {
//需要重写其newConstant方法,告诉ConstantPool如果当前变量
//不在常量池时如何初始化
@Override
protected AttributeKey<Object> newConstant(int id, String name) {
return new AttributeKey<Object>(id, name);
}
};
下面直接贴出ConstantPool
源码,其代码比较简单,相关说明写在注释里:
//ConstantPool
/**
* A pool of {@link Constant}s.
*
* @param <T> the type of the constant
*/
public abstract class ConstantPool<T extends Constant<T>> {
//常量池使用ConcurrentMap保存常量
private final ConcurrentMap<String, T> constants = PlatformDependent.newConcurrentHashMap();
//采用递增计数器生成常量ID
private final AtomicInteger nextId = new AtomicInteger(1);
/**
* Shortcut of {@link #valueOf(String) valueOf(firstNameComponent.getName() + "#" + secondNameComponent)}.
*/
//根据Class和String名称获取或生成并返回常量
public T valueOf(Class<?> firstNameComponent, String secondNameComponent) {
if (firstNameComponent == null) {
throw new NullPointerException("firstNameComponent");
}
if (secondNameComponent == null) {
throw new NullPointerException("secondNameComponent");
}
return valueOf(firstNameComponent.getName() + '#' + secondNameComponent);
}
/**
* Returns the {@link Constant} which is assigned to the specified {@code name}.
* If there's no such {@link Constant}, a new one will be created and returned.
* Once created, the subsequent calls with the same {@code name} will always return the previously created one
* (i.e. singleton.)
*
* @param name the name of the {@link Constant}
*/
//从常量池中获取指定名称的常量,如果不存在则调用
//newConstant方法生成新的常量
public T valueOf(String name) {
checkNotNullAndNotEmpty(name);
return getOrCreate(name);
}
/**
* Get existing constant by name or creates new one if not exists. Threadsafe
*
* @param name the name of the {@link Constant}
*/
//从常量池中获取常量,如果不包含该常量则新增一个并返回
private T getOrCreate(String name) {
//从Map中获取指定名称的常量
T constant = constants.get(name);
//如果为空,则新建一个新的常量,并放到常量池中
if (constant == null) {
//调用newConstant构造一个新的常量
final T tempConstant = newConstant(nextId(), name);
//这里使用putIfAbset,采用了类似懒汉模式单例的
//Double-Check机制,防止重复put
constant = constants.putIfAbsent(name, tempConstant);
//如果返回null,表示double-check检查失败,返回刚新建的
//常量对象
if (constant == null) {
return tempConstant;
}
}
//返回新建或池中原实例
return constant;
}
/**
* Returns {@code true} if a {@link AttributeKey} exists for the given {@code name}.
*/
//检查池中是否存在指定名称的常量
public boolean exists(String name) {
checkNotNullAndNotEmpty(name);
return constants.containsKey(name);
}
/**
* Creates a new {@link Constant} for the given {@code name} or fail with an
* {@link IllegalArgumentException} if a {@link Constant} for the given {@code name} exists.
*/
//返回指定名称的常量实例
public T newInstance(String name) {
checkNotNullAndNotEmpty(name);
return createOrThrow(name);
}
/**
* Creates constant by name or throws exception. Threadsafe
*
* @param name the name of the {@link Constant}
*/
//返回池中指定名称对象,如果没有则创建,这里如果putIfAbsent
//失败会抛出异常
private T createOrThrow(String name) {
T constant = constants.get(name);
if (constant == null) {
final T tempConstant = newConstant(nextId(), name);
constant = constants.putIfAbsent(name, tempConstant);
if (constant == null) {
return tempConstant;
}
}
throw new IllegalArgumentException(String.format("'%s' is already in use", name));
}
private static String checkNotNullAndNotEmpty(String name) {
ObjectUtil.checkNotNull(name, "name");
if (name.isEmpty()) {
throw new IllegalArgumentException("empty name");
}
return name;
}
//供子类重写的方法,用于在未找到指定name的常量时进行初始化
protected abstract T newConstant(int id, String name);
@Deprecated
public final int nextId() {
return nextId.getAndIncrement();
}
}
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