文件IO
java.io.File是用于操作文件或目录的类:
File file = new File("hello.txt");
实例化File时不关心路径的目标并不会去读取文件或目录. File类提供了一些有用的方法:
-
isFile(): 判断路径指向的是否为文件 -
createNewFile(): 当路径指向的文件不存在时创建一个空文件 -
exists(): 判断路径指向的文件是否存在 -
delete(): 删除路径指向的文件或目录 -
length(): 返回文件的长度 -
isDirectory(): 判断路径指向的是否为目录 -
mkdir(): 根据路径创建空目录, 父目录必须存在 -
mkdirs(): 根据路径创建空目录, 会创建必要的父目录 -
list(): 以String[]类型返回目录中所有文件名 -
listFiles(): 以File[]类型返回目录中所有文件
字符流
Java使用流来读写文件, 字符流用来读写文本文件. 所有的字符流类都在java.io包中.
读取文本文件:
File file = new File("a.txt");
FileReader fin = new FileReader(file);
BufferedReader reader = new BufferedReader(fin);
try {
String str = reader.readLine();
System.out.println(str);
}
catch (IOException e) {
e.printStackTrace();
}
finally {
reader.close();
fin.close();
}
FileReader也可以直接用文件名创建. new FileReader("a.txt").
写入文本文件:
FileWriter fout = new FileWriter("a.txt", true);
BufferedWriter writer = new BufferedWriter(fout);
try {
String str = "Hello World";
writer.write(str);
}
catch (IOException e) {
e.printStackTrace();
}
finally {
writer.close();
fout.close();
}
FileWriter的第二个参数为append, true代表在文件尾追加, 默认false代表清空文件重写.
字节流
字节流用于读写二进制文件, 其读写的数据以byte[]类型存储.
所有字节流类都在java.io包中.
读取文件:
File file = new File("a.in");
FileInputStream fin = new FileInputStream(file);
Byte[] buf = new Byte[512];
try {
fin.read(buf);
}
catch {IOException e} {
e.printStackTrace();
}
finally {
fin.close();
}
写文件:
File file = new File("a.in");
FileOutputStream fout = new FileOutputStream(file);
Byte[] buf = new Byte[512];
try {
// write sth in buf
fin.write(buf);
}
catch {IOException e} {
e.printStackTrace();
}
finally {
fout.close();
}
标准输入输出
java.lang.System对象中维护了3个标准流, 用于终端输入输出:
System.out, 标准输出流,PrintStream对象System.err: 标准错误流,PrintStream对象System.in: 标准输入流,FileInputStream对象
在需要的时候我们可以将它们重定向到文件.
重定向标准输出:
File file = new File("a.out");
FileOutputStream fout = new FileOutputStream(file);
PrintStream pout = new PrintStream(fout);
PrintStream stdout = System.out; // save
System.setOut(pout);
System.setOut(stdout); // recover
因为标准输入是字节流, 我们需要把它们转换成需要的类型才能使用:
int n;
byte[] buf = new byte[1024];
n = System.in.read(buf);
String s = new String(buf, 0, n);
System.out.println(s);
java.util.Scanner允许用迭代器的方式读取输入:
Scanner scanner = new Scanner(System.in);
while (scanner.hasNext()) {
// scanner.next(); // return Object
scanner.nextInt(); // return int
}
scanner可以直接将输入转换为内置类型使用很方便.
网络IO
TCP客户端
java.net.Socket是一个用作Tcp客户端的Socket. 从Socket中获得InputStream和OutputStream对象就可以与服务器通过Tcp连接通信了.
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.net.Socket;
public class TcpClient {
public static void main(String[] args) throws IOException {
Socket client = new Socket("127.0.0.1", 5000);
client.setSoTimeout(10000);
PrintStream out = new PrintStream(client.getOutputStream());
BufferedReader buf = new BufferedReader(new InputStreamReader(client.getInputStream()));
String[] msgs = {"你好", "世界"};
for (String msg : msgs) {
out.println(msg);
while (true) {
String echo = buf.readLine();
if (echo != null) {
System.out.println(echo);
break;
}
}
}
}
}
上述示例中发送完一条消息则进入轮询, 查看是否有服务端消息.
java.net.ServerSocket则是一个用作Tcp服务端的socket.
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
public class TcpServer implements Runnable {
private Socket client = null;
private String address;
public TcpServer(Socket client) {
this.client = client;
}
@Override
public void run() {
try {
String host = client.getInetAddress().toString();
String port = Integer.toString(client.getPort());
PrintStream out = new PrintStream(client.getOutputStream());
System.out.println("Get Connection");
BufferedReader buf = new BufferedReader(new InputStreamReader(client.getInputStream()));
address = host + ":" + port;
System.out.println("get connection from" + address);
while (true) {
String str = buf.readLine();
if (str != null) {
out.println(str);
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
ArrayList list = new ArrayList();
try {
ServerSocket socket = new ServerSocket(5000);
while (true) {
// accept a new connection
Socket client = socket.accept();
Thread thread = new Thread(new TcpServer(client));
list.add(thread);
thread.start();
}
}
catch (IOException ioe) {
ioe.printStackTrace();
}
}
}
上述示例是一个多线程服务器, ServerSocket监听5000端口. 当有客户端连接该端口时, socket.accept()将返回一个java.net.Socket对象.
创建一个线程持有Socket对象, 并与客户端进行通信.
UDP客户端
java.net.Datagram可以用作UDP客户端, 其接收到的数据报被封装为java.net.DatagramPacket.
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
public class UdpClient {
public static void main(String[] args) throws IOException {
DatagramSocket socket = new DatagramSocket();
String[] msgs = {"1", "2.3", "520"};
for (String msg : msgs) {
byte[] buf = msg.getBytes();
InetAddress addr = InetAddress.getByName("127.0.0.1");
DatagramPacket packet = new DatagramPacket(buf, buf.length, addr, 5000);
socket.send(packet);
while (true) {
byte[] recvBuf = new byte[256];
DatagramPacket recvPacket = new DatagramPacket(recvBuf, recvBuf.length);
socket.receive(recvPacket);
String echo = new String(recvPacket.getData(), 0, recvPacket.getLength());
if (echo != null) {
System.out.println(echo);
break;
}
}
}
}
}
DatagramSocket的send方法用于发送数据报, receive用于接收数据报.
UDP服务器
import java.io.IOException;
import java.net.DatagramSocket;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.Socket;
import java.util.ArrayList;
import java.util.concurrent.atomic.AtomicInteger;
public class UdpServer implements Runnable {
private DatagramSocket socket = null;
private DatagramPacket packet = null;
private String address;
public UdpServer(DatagramSocket socket, DatagramPacket packet) {
this.socket = socket;
this.packet = packet;
}
@Override
public void run() {
try {
String msg = new String(packet.getData());
UdpServer.sum += Double.parseDouble(msg);
UdpServer.count += 1;
int port = packet.getPort();
InetAddress address = packet.getAddress();
System.out.println("get msg from" + address);
String response = new String(recvPacket.getData(), 0, recvPacket.getLength());
DatagramPacket sendPacket = new DatagramPacket(response.getBytes(), response.getBytes().length, address, port);
socket.send(sendPacket);
}
catch (IOException ioe) {
ioe.printStackTrace();
}
}
public static void main(String[] args) {
AtomicInteger numThreads = new AtomicInteger(0);
ArrayList list = new ArrayList();
try {
DatagramSocket socket = new DatagramSocket(5000);
while (true) {
byte[] buf = new byte[100];
DatagramPacket packet = new DatagramPacket(buf, buf.length);
socket.receive(packet);
Thread thread = new Thread(new UdpServer(socket, packet));
list.add(thread);
thread.start();
numThreads.incrementAndGet();
}
}
catch (IOException ioe) {
ioe.printStackTrace();
}
}
}
因为UDP不需要维护连接, 服务端和客户端的socket是同样的.
上文是一个多线程UDP服务器, 不过所有线程持有同一个socket对象和要处理的数据报. 为了保证线程安全最好给socket加锁.