Java程序员的Golang入门指南(下)4.高级特性上面介绍的只是Golang的基本语法和特性，尽管像控制语句的条件不用圆括号、函数多返回值、switch-case默认break、函数闭包、集合切片等特性相比Java的确提高了开发效率，但这些在其他语言中也都有，并不是Golang能真正吸引人的地方。不仅是Golang，我们学习任何语言当然都是从基本语法特性着手，但学习时要不断地问自己：使这门语言区别于其他语言的”独到之处“在哪？这种独到之处往往反映了语言的设计思想、出发点、要解决的”痛点“，这才是一门语言或任何技术的立足之本。4.1 goroutinegoroutine使用go关键字来调用函数，也可以使用匿名函数。可以简单的把go关键字调用的函数想像成pthread_create。如果一个goroutine没有被阻塞，那么别的goroutine就不会得到执行。也就是说goroutine阻塞时，Golang会切换到其他goroutine执行，这是非常好的特性！Java对类似goroutine这种的协程没有原生支持，像Akka最害怕的就是阻塞。因为协程不等同于线程，操作系统不会帮我们完成“现场”保存和恢复，所以要实现goroutine这种特性，就要模拟操作系统的行为，保存方法或函数在协程“上下文切换”时的Context，当阻塞结束时才能正确地切换回来。像Kilim等协程库利用字节码生成，能够胜任，而Akka完全是运行时的。注意：如果你要真正的并发，需要调用runtime.GOMAXPROCS(CPU_NUM)设置。packagemainimport"fmt"funcmain() {gof("goroutine")gofunc(msgstring) {
fmt.Println(msg)
}("going")// Block main threadvarinputstringfmt.Scanln(&input;)
fmt.Println("done")
}funcf(msgstring) {
fmt.Println(msg)
}4.2 原子操作像Java一样，Golang支持很多CAS操作。运行结果是unsaftCnt可能小于200，因为unsafeCnt++在机器指令层面上不是一条指令，而可能是从内存加载数据到寄存器、执行自增运算、保存寄存器中计算结果到内存这三部分，所以不进行保护的话有些更新是会丢失的。packagemainimport("fmt""time""sync/atomic""runtime")funcmain() {// IMPORTANT!!!runtime.GOMAXPROCS(4)// thread-unsafevarunsafeCntint32=0fori :=0; i <10; i++ {gofunc() {fori :=0; i <20; i++ {
time.Sleep(time.Millisecond)
unsafeCnt++
}
}()
}
time.Sleep(time.Second)
fmt.Println("cnt: ", unsafeCnt)// CAS toolkitvarcntint32=0fori :=0; i <10; i++ {gofunc() {fori :=0; i <20; i++ {
time.Sleep(time.Millisecond)
atomic.AddInt32(&cnt;,1)
}
}()
}
time.Sleep(time.Second)
cntFinal := atomic.LoadInt32(&cnt;)
fmt.Println("cnt: ", cntFinal)
}神奇CAS的原理Golang的AddInt32()类似于Java中AtomicInteger.incrementAndGet()，其伪代码可以表示如下。二者的基本思想是一致的，本质上是乐观锁：首先，从内存位置M加载要修改的数据到寄存器A中；然后，修改数据并保存到另一寄存器B；最终，利用CPU提供的CAS指令（Java通过JNI调用到）用一条指令完成：1）A值与M处的原值比较；2）若相同则将B值覆盖到M处。若不相同，则CAS指令会失败，说明从内存加载到执行CAS指令这一小段时间内，发生了上下文切换，执行了其他线程的代码修改了M处的变量值。那么重新执行前面几个步骤再次尝试。ABA问题：即另一线程修改了M位置的数据，但是从原值改为C，又从C改回原值。这样上下文切换回来，CAS指令发现M处的值“未改变”（实际是改了两次，最后改回来了），所以CAS指令正常执行，不会失败。这种问题在Java中可以用AtomicStampedReference/AtomicMarkableReference解决。publicfinalintincrementAndGet() {for(;;) {intcurrent = get();intnext = current +1;if(compareAndSet(current, next))returnnext;
}
}4.3 Channel管道通过前面可以看到，尽管goroutine很方便很高效，但如果滥用的话很可能会导致并发安全问题。而Channel就是用来解决这个问题的，它是goroutine之间通信的桥梁，类似Actor模型中每个Actor的mailbox。多个goroutine要修改一个状态时，可以将请求都发送到一个Channel里，然后由一个goroutine负责顺序地修改状态。Channel默认是阻塞的，也就是说select时如果没有事件，那么当前goroutine会发生读阻塞。同理，Channel是有大小的，当Channel满了时，发送方会发生写阻塞。Channel这种阻塞的特性加上goroutine可以很容易就能实现生产者-消费者模式。用case可以给Channel设置阻塞的超时时间，避免一直阻塞。而default则使select进入无阻塞模式。packagemainimport("fmt""time")/**
* Output:
* received message: hello
* received message: world
*
* received from channel-1: Hello
* received from channel-2: World
*
* received message: hello
* Time out!
*
* Nothing received!
* received message: hello
* Nothing received!
* Nothing received!
* Nothing received!
* Nothing received!
* Nothing received!
* Nothing received!
* Nothing received!
* Nothing received!
* Nothing received!
* received message: world
* Nothing received!
* Nothing received!
* Nothing received!
*/funcmain() {
listenOnChannel()
selectTwoChannels()
blockChannelWithTimeout()
unblockChannel()
}funclistenOnChannel() {// Specify channel type and buffer sizechannel :=make(chanstring,5)gofunc() {
channel <-"hello"channel <-"world"}()fori :=0; i <2; i++ {
msg := <- channel
fmt.Println("received message: "+ msg)
}
}funcselectTwoChannels() {
c1 :=make(chanstring)
c2 :=make(chanstring)gofunc() {
time.Sleep(time.Second)
c1 <-"Hello"}()gofunc() {
time.Sleep(time.Second)
c2 <-"World"}()fori :=0; i <2; i++ {select{casemsg1 := <- c1:
fmt.Println("received from channel-1: "+ msg1)casemsg2 := <- c2:
fmt.Println("received from channel-2: "+ msg2)
}
}
}funcblockChannelWithTimeout() {
channel :=make(chanstring,5)gofunc() {
channel <-"hello"// Sleep 10 sectime.Sleep(time.Second *10)
channel <-"world"}()fori :=0; i <2; i++ {select{casemsg := <- channel:
fmt.Println("received message: "+ msg)// Set timeout 5 seccase<- time.After(time.Second *5):
fmt.Println("Time out!")
}
}
}funcunblockChannel() {
channel :=make(chanstring,5)gofunc() {
channel <-"hello"time.Sleep(time.Second *10)
channel <-"world"}()fori :=0; i <15; i++ {select{casemsg := <- channel:
fmt.Println("received message: "+ msg)default:
fmt.Println("Nothing received!")
time.Sleep(time.Second)
}
}
}4.4 缓冲流Golang的bufio包提供了方便的缓冲流操作，通过strings或网络IO得到流后，用bufio.NewReader/Writer()包装：缓冲区：Peek()或Read时，数据会从底层进入到缓冲区。缓冲区默认大小为4096字节。切片和拷贝：Peek()和ReadSlice()得到的都是切片（缓冲区数据的引用）而不是拷贝，所以更加节约空间。但是当缓冲区数据变化时，切片也会随之变化。而ReadBytes/String()得到的都是数据的拷贝，可以放心使用。Unicode支持：ReadRune()可以直接读取Unicode字符。有意思的是Golang中Unicode字符也要用单引号，这点与Java不同。分隔符：ReadSlice/Bytes/String()得到的包含分隔符，bufio不会自动去掉。Writer：对应地，Writer提供了WriteBytes/String/Rune。undo方法：可以将读出的字节再放回到缓冲区，就像什么都没发生一样。packagemainimport("fmt""strings""bytes""bufio")/**
* Buffered: 0
* Buffered after peek: 7
* ABCDE
* AxCDE
*
* abcdefghijklmnopqrst 20
* uvwxyz1234567890 16
* 0 EOF
*
* "ABC "
* "DEF "
* "GHI"
*
* "ABC "
* "DEF "
* "GHI"
*
* read unicode=[你], size=[3]
* read unicode=[好], size=[3]
* read(after undo) unicode=[好], size=[3]
*
* Available: 4096
* Buffered: 0
* Available after write: 4088
* Buffered after write: 8
* Buffer after write: ""
* Available after flush: 4096
* Buffered after flush: 0
* Buffer after flush: "ABCDEFGH"
*
* Hello,世界！
*/funcmain() {
testPeek()
testRead()
testReadSlice()
testReadBytes()
testReadUnicode()
testWrite()
testWriteByte()
}functestPeek() {
r := strings.NewReader("ABCDEFG")
br := bufio.NewReader(r)
fmt.Printf("Buffered: %d\n", br.Buffered())
p, _ := br.Peek(5)
fmt.Printf("Buffered after peek: %d\n", br.Buffered())
fmt.Printf("%s\n", p)
p[1] ='x'p, _ = br.Peek(5)
fmt.Printf("%s\n", p)
}functestRead() {
r := strings.NewReader("abcdefghijklmnopqrstuvwxyz1234567890")
br := bufio.NewReader(r)
b :=make([]byte,20)
n, err := br.Read(b)
fmt.Printf("%-20s %-2v %v\n", b[:n], n, err)
n, err = br.Read(b)
fmt.Printf("%-20s %-2v %v\n", b[:n], n, err)
n, err = br.Read(b)
fmt.Printf("%-20s %-2v %v\n", b[:n], n, err)
}functestReadSlice() {
r := strings.NewReader("ABC DEF GHI")
br := bufio.NewReader(r)
w, _ := br.ReadSlice(' ')
fmt.Printf("%q\n", w)
w, _ = br.ReadSlice(' ')
fmt.Printf("%q\n", w)
w, _ = br.ReadSlice(' ')
fmt.Printf("%q\n", w)
}functestReadBytes() {
r := strings.NewReader("ABC DEF GHI")
br := bufio.NewReader(r)
w, _ := br.ReadBytes(' ')
fmt.Printf("%q\n", w)
w, _ = br.ReadSlice(' ')
fmt.Printf("%q\n", w)
s, _ := br.ReadString(' ')
fmt.Printf("%q\n", s)
}functestReadUnicode() {
r := strings.NewReader("你好，世界！")
br := bufio.NewReader(r)
c, size, _ := br.ReadRune()
fmt.Printf("read unicode=[%c], size=[%v]\n", c, size)
c, size, _ = br.ReadRune()
fmt.Printf("read unicode=[%c], size=[%v]\n", c, size)
br.UnreadRune()
c, size, _ = br.ReadRune()
fmt.Printf("read(after undo) unicode=[%c], size=[%v]\n", c, size)
}functestWrite() {
b := bytes.NewBuffer(make([]byte,0))
bw := bufio.NewWriter(b)
fmt.Printf("Available: %d\n", bw.Available())
fmt.Printf("Buffered: %d\n", bw.Buffered())
bw.WriteString("ABCDEFGH")
fmt.Printf("Available after write: %d\n", bw.Available())
fmt.Printf("Buffered after write: %d\n", bw.Buffered())
fmt.Printf("Buffer after write: %q\n", b)
bw.Flush()
fmt.Printf("Available after flush: %d\n", bw.Available())
fmt.Printf("Buffered after flush: %d\n", bw.Buffered())
fmt.Printf("Buffer after flush: %q\n", b)
}functestWriteByte() {
b := bytes.NewBuffer(make([]byte,0))
bw := bufio.NewWriter(b)
bw.WriteByte('H')
bw.WriteByte('e')
bw.WriteByte('l')
bw.WriteByte('l')
bw.WriteByte('o')
bw.WriteString(",")
bw.WriteRune('世')
bw.WriteRune('界')
bw.WriteRune('！')
bw.Flush()
fmt.Println(b)
}4.5 并发控制sync包中的WaitGroup是个很有用的类，类似信号量。wg.Add()和Done()能够加减WaitGroup（信号量）的值，而Wait()会挂起当前线程直到信号量变为0。下面的例子用WaitGroup的值表示正在运行的goroutine数量。在goroutine中，用defer Done()确保goroutine正常或异常退出时，WaitGroup都能减一。packagemainimport("fmt""sync")/**
* I'm waiting all goroutines on wg done
* I'm done=[0]
* I'm done=[1]
* I'm done=[2]
* I'm done=[3]
* I'm done=[4]
* I'm done=[5]
* I'm done=[6]
* I'm done=[7]
* I'm done=[8]
* I'm done=[9]
*/funcmain() {varwg sync.WaitGroupfori :=0; i <10; i++ {
wg.Add(1)gofunc(idint) {deferwg.Done()
fmt.Printf("I'm done=[%d]\n", id)
}(i)
}
fmt.Println("I'm waiting all goroutines on wg done")
wg.Wait()
}4.6 网络编程Golang的net包的抽象层次还是挺高的，用不了几行代码就能实现一个简单的TCP或HTTP服务端了。4.6.1 Socket编程packagemainimport("net""fmt""io")/**
* Starting the server
* Accept the connection: 127.0.0.1:14071
* Warning: End of data EOF
*/funcmain() {
listener, err := net.Listen("tcp","127.0.0.1:12345")iferr !=nil{panic("error listen: "+ err.Error())
}
fmt.Println("Starting the server")for{
conn, err := listener.Accept()iferr !=nil{panic("error accept: "+ err.Error())
}
fmt.Println("Accept the connection: ", conn.RemoteAddr())goechoServer(conn)
}
}funcechoServer(conn net.Conn) {
buf :=make([]byte,1024)deferconn.Close()for{
n, err := conn.Read(buf)switcherr {casenil:
conn.Write(buf[0:n])caseio.EOF:
fmt.Printf("Warning: End of data %s\n", err)returndefault:
fmt.Printf("Error: read data %s\n", err)return}
}
}4.6.2 Http服务器packagemainimport("fmt""log""net/http")funcmain() {
http.HandleFunc("/hello", handleHello)
fmt.Println("serving on http://localhost:7777/hello")
log.Fatal(http.ListenAndServe("localhost:7777",nil))
}funchandleHello(w http.ResponseWriter, req *http.Request) {
log.Println("serving", req.URL)
fmt.Fprintln(w,"Hello, world!")
}5.结束语5.1 Golang初体验Golang的某些语法的确很简洁，像行尾无分号、条件语句无括号、类型推断、函数多返回值、异常处理、原生协程支持、DuckType继承等，尽管很多并不是Golang首创，但结合到一起写起来还是很舒服的。当然Golang也有让人“不爽”的地方。像变量和函数中的类型声明写在后面简直是“反人类”！同样是颠覆，switch的case默认会break就很实用。另外，因为Golang主要还是想替代C做系统开发，所以像类啊、包啊还是能看到C的影子，例如类声明只有成员变量而不会包含方法实现等，支持全局函数等，所以有时看到aaa.bbb()还是有点迷糊，不知道aaa是包名还是实例名。5.2 如何学习一门语言当我们谈到学习英语时，想到的可能是背单词、学语法、练习听说读写。对于编程语言来说，背单词（关键字）、学语法（语法规则）少不了，可听说读写只剩下了“写”，因为我们说话的对象是“冷冰冰”的计算机。所以唯一的捷径就是“写”，不断地练习！此外，学的语言多了也能总结出一些规律。首先是基础语法，包括了变量和常量、控制语句、函数、集合、OOP、异常处理、控制台输入输出、包管理等。然后是高级特性就差别比较大了。专注高并发的语言就要看并发方面的特性，专注OOP的语言就要看有哪些抽象层次更高的特性等等。还是那句话，基础语言只能说我们会用，而能够区别一门语言的高级特性才是它的根本和灵魂，也是我们要着重学习和领悟的地方。