Java NIO：异步通信与效率提升

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"Java 异步通信是Java编程中一种重要的设计模式，它允许程序在不等待某个操作完成的情况下继续执行其他任务。这种方式提高了应用程序的响应性和效率，尤其是在处理大量并发请求时。Java的异步通信主要通过回调、事件驱动或者Future模式等方式实现。在Java中，异步通信通常与非阻塞I/O（Non-blocking I/O）结合使用，特别是Java NIO（New I/O）库在Java 1.4版本引入后，提供了对异步I/O的强大支持。Java NIO不仅补充了传统I/O的功能，还引入了选择器（Selectors）和通道（Channels）等概念，使得开发者能够更高效地管理多个输入/输出流。 1. 对于有返回值的请求，例如`makeString`，在异步模型中，调用者并不会立即得到结果，而是会得到一个表示操作的句柄或Future对象。这个句柄可以用来在稍后的时间获取操作的结果，而不会阻塞当前线程。 2. 并发处理多个请求是异步通信的核心特性。在描述中提到，可以通过队列来管理和调度请求，如使用FIFO（先进先出）的Queue。这确保了请求的顺序执行，但也可以通过修改队列实现如优先级队列，让高优先级的任务先被处理。 3. Java NIO还允许跨越机器的异步通信，通过RMI（Remote Method Invocation）技术，使得在分布式系统中的异步调用成为可能。这样，一个服务可以调用远程机器上的方法，而不必等待响应，从而提高整体系统的可伸缩性。 4. 分离调用句柄和真实数据的实现是解耦合的一种方式，它降低了系统各部分之间的依赖。例如，一个线程发起异步请求后，只需持有句柄，而实际的数据处理则在另一个线程中进行。这种分离也有助于实现回调函数或者事件监听器，它们可以在数据准备好后被通知。 5. 异步通信的一个关键好处是它可以快速返回控制给调用者，即使实际的操作尚未完成。这显著提高了系统响应时间，特别是在处理I/O密集型任务时，因为I/O操作通常比CPU计算更耗时。书中《Java NIO》由Ron Hitchens撰写，详细探讨了Java 1.4引入的新I/O功能，并提供了实例来展示如何利用这些特性解决实际的I/O问题，以提升代码效率。这本书涵盖了如选择器、通道、缓冲区等多个主题，帮助读者理解何时应该使用新的NIO API，以及何时旧的1.3 I/O API更适合特定的应用场景。总结起来，Java的异步通信和NIO机制是构建高性能、高并发系统的关键工具，它们提供了更灵活和高效的I/O处理方式，对于现代Java开发者来说是不可或缺的知识点。"

Java NIO

1.3 Getting to the Good Stuff

Most of the development effort that goes into operating systems is targeted at improving I/O

performance. Lots of very smart people toil very long hours perfecting techniques for

schlepping data back and forth. Operating-system vendors expend vast amounts of time and

money seeking a competitive advantage by beating the other guys in this or that published

benchmark.

Today's operating systems are modern marvels of software engineering (OK, some are more

marvelous than others), but how can the Java programmer take advantage of all this wizardry

and still remain platform-independent? Ah, yet another example of the TANSTAAFL

principle.

The JVM is a double-edged sword. It provides a uniform operating environment that shelters

the Java programmer from most of the annoying differences between operating-system

environments. This makes it faster and easier to write code because platform-specific

idiosyncrasies are mostly hidden. But cloaking the specifics of the operating system means

that the jazzy, wiz-bang stuff is invisible too.

What to do? If you're a developer, you could write some native code using the Java Native

Interface (JNI) to access the operating-system features directly. Doing so ties you to a specific

operating system (and maybe a specific version of that operating system) and exposes the

JVM to corruption or crashes if your native code is not 100% bug free. If you're an operating-

system vendor, you could write native code and ship it with your JVM implementation to

provide these features as a Java API. But doing so might violate the license you signed to

provide a conforming JVM. Sun took Microsoft to court about this over the JDirect package

which, of course, worked only on Microsoft systems. Or, as a last resort, you could turn to

another language to implement performance-critical applications.

The

java.nio

package provides new abstractions to address this problem. The Channel and

Selector classes in particular provide generic APIs to I/O services that were not reachable

prior to JDK 1.4. The TANSTAAFL principle still applies: you won't be able to access every

feature of every operating system, but these new classes provide a powerful new framework

that encompasses the high-performance I/O features commonly available on commercial

operating systems today. Additionally, a new Service Provider Interface (SPI) is provided in

java.nio.channels.spi

that allows you to plug in new types of channels and selectors

without violating compliance with the specifications.

With the addition of NIO, Java is ready for serious business, entertainment, scientific and

academic applications in which high-performance I/O is essential.

The JDK 1.4 release contains many other significant improvements in addition to NIO. As of

1.4, the Java platform has reached a high level of maturity, and there are few application areas

remaining that Java cannot tackle. A great guide to the full spectrum of JDK features in 1.4 is

Java In A Nutshell, Fourth Edition by David Flanagan (O'Reilly).

There Ain't No Such Thing As A Free Lunch.

Java NIO

1.4 I/O Concepts

The Java platform provides a rich set of I/O metaphors. Some of these metaphors are more

abstract than others. With all abstractions, the further you get from hard, cold reality,

the tougher it becomes to connect cause and effect. The NIO packages of JDK 1.4 introduce

a new set of abstractions for doing I/O. Unlike previous packages, these are focused on

shortening the distance between abstraction and reality. The NIO abstractions have very real

and direct interactions with real-world entities. Understanding these new abstractions and, just

as importantly, the I/O services they interact with, is key to making the most of I/O-intensive

Java applications.

This book assumes that you are familiar with basic I/O concepts. This section provides

a whirlwind review of some basic ideas just to lay the groundwork for the discussion of how

the new NIO classes operate. These classes model I/O functions, so it's necessary to grasp

how things work at the operating-system level to understand the new I/O paradigms.

In the main body of this book, it's important to understand the following topics:

•

Buffer handling

• Kernel versus user space

• Virtual memory

• Paging

• File-oriented versus stream I/O

•

Multiplexed I/O (readiness selection)

1.4.1 Buffer Handling

Buffers, and how buffers are handled, are the basis of all I/O. The very term "input/output"

means nothing more than moving data in and out of buffers.

Processes perform I/O by requesting of the operating system that data be drained from

a buffer (write) or that a buffer be filled with data (read). That's really all it boils down to. All

data moves in or out of a process by this mechanism. The machinery inside the operating

system that performs these transfers can be incredibly complex, but conceptually, it's very

straightforward.

Figure 1-1 shows a simplified logical diagram of how block data moves from an external

source, such as a disk, to a memory area inside a running process. The process requests that

its buffer be filled by making the read( ) system call. This results in the kernel issuing

a command to the disk controller hardware to fetch the data from disk. The disk controller

writes the data directly into a kernel memory buffer by DMA without further assistance from

the main CPU. Once the disk controller finishes filling the buffer, the kernel copies the data

from the temporary buffer in kernel space to the buffer specified by the process when it

requested the read( ) operation.

Java NIO

Figure 1-1. Simplified I/O buffer handling

This obviously glosses over a lot of details, but it shows the basic steps involved.

Note the concepts of user space and kernel space in Figure 1-1. User space is where regular

processes live. The JVM is a regular process and dwells in user space. User space is

a nonprivileged area: code executing there cannot directly access hardware devices, for

example. Kernel space is where the operating system lives. Kernel code has special privileges:

it can communicate with device controllers, manipulate the state of processes in user space,

etc. Most importantly, all I/O flows through kernel space, either directly (as decsribed here) or

indirectly (see Section 1.4.2).

When a process requests an I/O operation, it performs a system call, sometimes known as

a trap, which transfers control into the kernel. The low-level open( ), read( ), write( ), and

close( ) functions so familiar to C/C++ coders do nothing more than set up and perform the

appropriate system calls. When the kernel is called in this way, it takes whatever steps are

necessary to find the data the process is requesting and transfer it into the specified buffer in

user space. The kernel tries to cache and/or prefetch data, so the data being requested by the

process may already be available in kernel space. If so, the data requested by the process is

copied out. If the data isn't available, the process is suspended while the kernel goes about

bringing the data into memory.

Looking at Figure 1-1, it's probably occurred to you that copying from kernel space to the

final user buffer seems like extra work. Why not tell the disk controller to send it directly to

the buffer in user space? There are a couple of problems with this. First, hardware is usually

not able to access user space directly.

Second, block-oriented hardware devices such as disk

controllers operate on fixed-size data blocks. The user process may be requesting an oddly

sized or misaligned chunk of data. The kernel plays the role of intermediary, breaking down

and reassembling data as it moves between user space and storage devices.

1.4.1.1 Scatter/gather

Many operating systems can make the assembly/disassembly process even more efficient. The

notion of scatter/gather allows a process to pass a list of buffer addresses to the operating

system in one system call. The kernel can then fill or drain the multiple buffers in sequence,

scattering the data to multiple user space buffers on a read, or gathering from several buffers

on a write (Figure 1-2).

There are many reasons for this, all of which are beyond the scope of this book. Hardware devices usually cannot directly use virtual memory

addresses.

Java NIO

Figure 1-4. Memory pages

1.4.3 Memory Paging

To support the second attribute of virtual memory (having an addressable space larger than

physical memory), it's necessary to do virtual memory paging (often referred to as swapping,

though true swapping is done at the process level, not the page level). This is a scheme

whereby the pages of a virtual memory space can be persisted to external disk storage to make

room in physical memory for other virtual pages. Essentially, physical memory acts as a

cache for a paging area, which is the space on disk where the content of memory pages is

stored when forced out of physical memory.

Figure 1-5 shows virtual pages belonging to four processes, each with its own virtual memory

space. Two of the five pages for Process A are loaded into memory; the others are stored on

disk.

Figure 1-5. Physical memory as a paging-area cache

Aligning memory page sizes as multiples of the disk block size allows the kernel to issue

direct commands to the disk controller hardware to write memory pages to disk or reload

them when needed. It turns out that all disk I/O is done at the page level. This is the only way

data ever moves between disk and physical memory in modern, paged operating systems.

Modern CPUs contain a subsystem known as the Memory Management Unit (MMU). This

device logically sits between the CPU and physical memory. It contains the mapping

information needed to translate virtual addresses to physical memory addresses. When

the CPU references a memory location, the MMU determines which page the location resides

in (usually by shifting or masking the bits of the address value) and translates that virtual page

number to a physical page number (this is done in hardware and is extremely fast). If there is

no mapping currently in effect between that virtual page and a physical memory page, the

MMU raises a page fault to the CPU.

A page fault results in a trap, similar to a system call, which vectors control into the kernel

along with information about which virtual address caused the fault. The kernel then takes

steps to validate the page. The kernel will schedule a pagein operation to read the content of

the missing page back into physical memory. This often results in another page being stolen

to make room for the incoming page. In such a case, if the stolen page is dirty (changed since

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