java网络编程学习手记.docx

Writing a single byte at a time is often inefficient. For example, every TCP segment that goes out your Ethernet card contains at least 40 bytes of overhead for routing and error correction. If each byte is sent by itself, you may be stuffing（充满，塞满） the network with 41 times more data than you think you are! Consequently, most TCP/IP implementations buffer data to some extent. That is, they accumulate bytes in memory and send them to their eventual destination only when a certain number have accumulated or a certain amount of time has passed. However, if you have more than one byte ready to go, its not a bad idea to send them all at once. Using write(byte data) or write(byte data, int offset, int length) is normally much faster than writing all the components of the data array one at a time. For instance, heres an implementation of the generateCharacters() method that sends a line at a time by packing a complete line into a byte array:The .InetAddress class is Javas high-level representation of an IP address, both IPv4 and IPv6. It is used by most of the other networking classes, including Socket, ServerSocket, URL, DatagramSocket, DatagramPacket, and more. Generally, it includes both a hostname and an IP address.public class InetAddress extends Object implements SerializableInetAddress类中没有公有的构造函数，但是它有3个静态方法There are no public constructors in the InetAddress class. However, InetAddress has three static methods that return suitably initialized InetAddress objects given a little information. They are:public static InetAddress getByName(String hostName) throws UnknownHostExceptionpublic static InetAddress getAllByName(String hostName) throws UnknownHostExceptionpublic static InetAddress getLocalHost( ) throws UnknownHostExceptionSince DNS lookups can be relatively expensive (on the order of several seconds for a request that has to go through several intermediate servers, or one thats trying to resolve an unreachable host) the InetAddress class caches the results of lookups. Once it has the address of a given host, it wont look it up again, even if you create a new InetAddress object for the same host. As long as IP addresses dont change while your program is running, this is not a problem.InetAddress类缓存了查找的结果下面程序只对java 1.4以前的版本有效Example 6-2. A program that prints the address of 7import .*;public class OReillyByAddress public static void main (String args) try InetAddress address = InetAddress.getByName(7); System.out.println(address); catch (UnknownHostException ex) System.out.println(Could not find 7); Heres the result in Java 1.3 and earlier:% java OReillyByAddress /7When you call getByName( ) with an IP address string as an argument, it creates an InetAddress object for the requested IP address without checking with DNS. This means its possible to create InetAddress objects for hosts that dont really exist and that you cant connect to. The hostname of an InetAddress object created from a string containing an IP address is initially set to that string. A DNS lookup for the actual hostname is performed only when the hostname is requested, either explicitly via getHostName( ) or implicitly through toString( ). Thats how was determined from the dotted quad address 7. If at the time the hostname is requested and a DNS lookup is finally performed the host with the specified IP address cant be found, then the hostname remains the original dotted quad string. However, no UnknownHostException is thrown.The toString( ) method in Java 1.4 behaves a little differently than in earlier versions. It does not do a reverse name lookup; thus, the host is not printed unless it is already known, either because it was provided as an argument to the factory method or because getHostName( ) was invoked. In Java 1.4, Example 6-2 produces this output:/7import .*; import .*;public class Test public static void main(String args) String add=; try InetAddress address=InetAddress.getByName(add); InetAddress addr=InetAddress.getByName(0); InetAddress alladdress=InetAddress.getAllByName(add); System.out.println(address); System.out.println(addr); for(int i=0;ialladdress.length;i+) System.out.println(alladdressi); catch(UnknownHostException e) e.getMessage(); /此程序获取域名下的所有IPpublic class Test public static void main(String args) String add=; try InetAddress address=InetAddress.getByName(add); InetAddress addr=InetAddress.getByName(0); System.out.println(address); System.out.println(addr); catch(UnknownHostException e) e.getMessage(); import .*;public class Test public static void main(String args) try/获取本机的名称，ip InetAddress add=InetAddress.getLocalHost(); System.out.println(add); catch(UnknownHostException e) e.getMessage(); The .URL class is an abstraction of a Uniform Resource Locator such as / or /pub/. It extends java.lang.Object, and it is a final class that cannot be subclassed. Rather than relying on inheritance to configure instances for different kinds of URLs, it uses the strategy design pattern. Protocol handlers are the strategies, and the URL class itself forms the context through which the different strategies are selected:public final class URL extends Object implements SerializableAlthough storing a URL as a string would be trivial(琐碎的), it is helpful to think of URLs as objects with fields that include the scheme (a.k.a. 亦称the protocol), hostname, port, path, query string, and fragment identifier (a.k.a. the ref), each of whi