有时需要在网上搜集信息，如果需要搜集的是获取方法单一而人工搜集费时费力的信息，比如统计一个网站每个月发了多少篇文章、用了哪些标签，为自然语言处理项目搜集语料，或者为模式识别项目搜集图片等等，就需要爬虫程序来完成这样的任务。而且搜索引擎必不可少的组件之一也是网络爬虫。
很多网络爬虫都是用Python，Java或C#实现的。我这里给出的是Java版本的爬虫程序。为了节省时间和空间，我把程序限制在只扫描本博客地址下的网页（也就是http://johnhan.net/但不包括http://johnhany.net/wp-content/下的内容），并从网址中统计出所用的所有标签。只要稍作修改，去掉代码里的限制条件就能作为扫描整个网络的程序使用。或者对输出格式稍作修改，可以作为生成博客sitemap的工具。
代码也可以在这里下载：johnhany/WPCrawler。
环境需求
我的开发环境是Windows7 + Eclipse。
需要XAMPP提供通过url访问MySQL数据库的端口。
还要用到三个开源的Java类库：
Apache HttpComponents 4.3 提供HTTP接口，用来向目标网址提交HTTP请求，以获取网页的内容；
HTML Parser 2.0 用来解析网页，从DOM节点中提取网址链接；
MySQL Connector/J 5.1.27 连接Java程序和MySQL，然后就可以用Java代码操作数据库。
代码
代码位于三个文件中，分别是：crawler.java，httpGet.java和parsePage.java。包名为net.johnhany.wpcrawler。
crawler.java
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package
net.johnhany.wpcrawler;
import
java.sql.Connection;
import
java.sql.DriverManager;
import
java.sql.ResultSet;
import
java.sql.SQLException;
import
java.sql.Statement;
public
class
crawler {
public
static
void
main(String args[]) throws
Exception {
String frontpage = "http://johnhany.net/";
Connection conn = null;
//connect the MySQL database
try
{
Class.forName("com.mysql.jdbc.Driver");
String dburl = "jdbc:mysql://localhost:3306?useUnicode=true&characterEncoding=utf8";
conn = DriverManager.getConnection(dburl, "root", "");
System.out.println("connection built");
} catch
(SQLException e) {
e.printStackTrace();
} catch
(ClassNotFoundException e) {
e.printStackTrace();
}
String sql = null;
String url = frontpage;
Statement stmt = null;
ResultSet rs = null;
int
count = 0;
if(conn != null) {
//create database and table that will be needed
try
{
sql = "CREATE DATABASE IF NOT EXISTS crawler";
stmt = conn.createStatement();
stmt.executeUpdate(sql);
sql = "USE crawler";
stmt = conn.createStatement();
stmt.executeUpdate(sql);
sql = "create table if not exists record (recordID int(5) not null auto_increment, URL text not null, crawled tinyint(1) not null, primary key (recordID)) engine=InnoDB DEFAULT CHARSET=utf8";
stmt = conn.createStatement();
stmt.executeUpdate(sql);
sql = "create table if not exists tags (tagnum int(4) not null auto_increment, tagname text not null, primary key (tagnum)) engine=InnoDB DEFAULT CHARSET=utf8";
stmt = conn.createStatement();
stmt.executeUpdate(sql);
} catch
(SQLException e) {
e.printStackTrace();
}
//crawl every link in the database
while(true) {
//get page content of link "url"
httpGet.getByString(url,conn);
count++;
//set boolean value "crawled" to true after crawling this page
sql = "UPDATE record SET crawled = 1 WHERE URL = '"
+ url + "'";
stmt = conn.createStatement();
if(stmt.executeUpdate(sql) > 0) {
//get the next page that has not been crawled yet
sql = "SELECT * FROM record WHERE crawled = 0";
stmt = conn.createStatement();
rs = stmt.executeQuery(sql);
if(rs.next()) {
url = rs.getString(2);
}else
{
//stop crawling if reach the bottom of the list
break;
}
//set a limit of crawling count
if(count > 1000
|| url == null) {
break;
}
}
}
conn.close();
conn = null;
System.out.println("Done.");
System.out.println(count);
}
}
}
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httpGet.java
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package
net.johnhany.wpcrawler;
import
java.io.IOException;
import
java.sql.Connection;
import
org.apache.http.HttpEntity;
import
org.apache.http.HttpResponse;
import
org.apache.http.client.ClientProtocolException;
import
org.apache.http.client.ResponseHandler;
import
org.apache.http.client.methods.HttpGet;
import
org.apache.http.impl.client.CloseableHttpClient;
import
org.apache.http.impl.client.HttpClients;
import
org.apache.http.util.EntityUtils;
public
class
httpGet {
public
final
static
void
getByString(String url, Connection conn) throws
Exception {
CloseableHttpClient httpclient = HttpClients.createDefault();
try
{
HttpGet httpget = new
HttpGet(url);
System.out.println("executing request "
+ httpget.getURI());
ResponseHandlernew
ResponseHandler
public
String handleResponse(
final
HttpResponse response) throws
ClientProtocolException, IOException {
int
status = response.getStatusLine().getStatusCode();
if
(status >= 200
&& status < 300) {
HttpEntity entity = response.getEntity();
return
entity != null
? EntityUtils.toString(entity) : null;
} else
{
throw
new
ClientProtocolException("Unexpected response status: "
+ status);
}
}
};
String responseBody = httpclient.execute(httpget, responseHandler);
/*
//print the content of the page
System.out.println("----------------------------------------");
System.out.println(responseBody);
System.out.println("----------------------------------------");
*/
parsePage.parseFromString(responseBody,conn);
} finally
{
httpclient.close();
}
}
}
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parsePage.java
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package
net.johnhany.wpcrawler;
import
java.sql.Connection;
import
java.sql.PreparedStatement;
import
java.sql.ResultSet;
import
java.sql.SQLException;
import
java.sql.Statement;
import
org.htmlparser.Node;
import
org.htmlparser.Parser;
import
org.htmlparser.filters.HasAttributeFilter;
import
org.htmlparser.tags.LinkTag;
import
org.htmlparser.util.NodeList;
import
org.htmlparser.util.ParserException;
import
java.net.URLDecoder;
public
class
parsePage {
public
static
void
parseFromString(String content, Connection conn) throws
Exception {
Parser parser = new
Parser(content);
HasAttributeFilter filter = new
HasAttributeFilter("href");
try
{
NodeList list = parser.parse(filter);
int
count = list.size();
//process every link on this page
for(int
i=0; i
Node node = list.elementAt(i);
if(node instanceof
LinkTag) {
LinkTag link = (LinkTag) node;
String nextlink = link.extractLink();
String mainurl = "http://johnhany.net/";
String wpurl = mainurl + "wp-content/";
//only save page from "http://johnhany.net"
if(nextlink.startsWith(mainurl)) {
String sql = null;
ResultSet rs = null;
PreparedStatement pstmt = null;
Statement stmt = null;
String tag = null;
//do not save any page from "wp-content"
if(nextlink.startsWith(wpurl)) {
continue;
}
try
{
//check if the link already exists in the database
sql = "SELECT * FROM record WHERE URL = '"
+ nextlink + "'";
stmt = conn.createStatement(ResultSet.TYPE_FORWARD_ONLY,ResultSet.CONCUR_UPDATABLE);
rs = stmt.executeQuery(sql);
if(rs.next()) {
}else
{
//if the link does not exist in the database, insert it
sql = "INSERT INTO record (URL, crawled) VALUES ('"
+ nextlink + "',0)";
pstmt = conn.prepareStatement(sql, Statement.RETURN_GENERATED_KEYS);
pstmt.execute();
System.out.println(nextlink);
//use substring for better comparison performance
nextlink = nextlink.substring(mainurl.length());
//System.out.println(nextlink);
if(nextlink.startsWith("tag/")) {
tag = nextlink.substring(4, nextlink.length()-1);
//decode in UTF-8 for Chinese characters
tag = URLDecoder.decode(tag,"UTF-8");
sql = "INSERT INTO tags (tagname) VALUES ('"
+ tag + "')";
pstmt = conn.prepareStatement(sql, Statement.RETURN_GENERATED_KEYS);
//if the links are different from each other, the tags must be different
//so there is no need to check if the tag already exists
pstmt.execute();
}
}
} catch
(SQLException e) {
//handle the exceptions
System.out.println("SQLException: "
+ e.getMessage());
System.out.println("SQLState: "
+ e.getSQLState());
System.out.println("VendorError: "
+ e.getErrorCode());
} finally
{
//close and release the resources of PreparedStatement, ResultSet and Statement
if(pstmt != null) {
try
{
pstmt.close();
} catch
(SQLException e2) {}
}
pstmt = null;
if(rs != null) {
try
{
rs.close();
} catch
(SQLException e1) {}
}
rs = null;
if(stmt != null) {
try
{
stmt.close();
} catch
(SQLException e3) {}
}
stmt = null;
}
}
}
}
} catch
(ParserException e) {
e.printStackTrace();
}
}
}
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程序原理
所谓“互联网”，是网状结构，任意两个节点间都有可能存在路径。爬虫程序对互联网的扫描，在图论角度来讲，就是对有向图的遍历（链接是从一个网页指向另一个网页，所以是有向的）。常见的遍历方法有深度优先和广度优先两种。相关理论知识可以参考树的遍历：这里和这里。我的程序采用的是广度优先方式。
程序从crawler.java的main()开始运行。
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Class.forName("com.mysql.jdbc.Driver");
String dburl = "jdbc:mysql://localhost:3306?useUnicode=true&characterEncoding=utf8";
conn = DriverManager.getConnection(dburl, "root", "");
System.out.println("connection built");
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首先，调用DriverManager连接MySQL服务。这里使用的是XAMPP的默认MySQL端口3306，端口值可以在XAMPP主界面看到：
Apache和MySQL都启动之后，在浏览器地址栏输入“http://localhost/phpmyadmin/”就可以看到数据库了。等程序运行完之后可以在这里检查一下运行是否正确。
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sql = "CREATE DATABASE IF NOT EXISTS crawler";
stmt = conn.createStatement();
stmt.executeUpdate(sql);
sql = "USE crawler";
stmt = conn.createStatement();
stmt.executeUpdate(sql);
sql = "create table if not exists record (recordID int(5) not null auto_increment, URL text not null, crawled tinyint(1) not null, primary key (recordID)) engine=InnoDB DEFAULT CHARSET=utf8";
stmt = conn.createStatement();
stmt.executeUpdate(sql);
sql = "create table if not exists tags (tagnum int(4) not null auto_increment, tagname text not null, primary key (tagnum)) engine=InnoDB DEFAULT CHARSET=utf8";
stmt = conn.createStatement();
stmt.executeUpdate(sql);
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连接好数据库后，建立一个名为“crawler”的数据库，在库里建两个表，一个叫“record”，包含字段“recordID”，“URL”和“crawled”，分别记录地址编号、链接地址和地址是否被扫描过；另一个叫“tags”，包含字段“tagnum”和“tagname”，分别记录标签编号和标签名。
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while(true) {
httpGet.getByString(url,conn);
count++;
sql = "UPDATE record SET crawled = 1 WHERE URL = '"
+ url + "'";
stmt = conn.createStatement();
if(stmt.executeUpdate(sql) > 0) {
sql = "SELECT * FROM record WHERE crawled = 0";
stmt = conn.createStatement();
rs = stmt.executeQuery(sql);
if(rs.next()) {
url = rs.getString(2);
}else
{
break;
}
}
}
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接着在一个while循环内依次处理表record内的每个地址。每次处理时，把地址url传递给httpGet.getByString()，然后在表record中把crawled改为true，表明这个地址已经处理过。然后寻找下一个crawled为false的地址，继续处理，直到处理到表尾。
这里需要注意的细节是，执行executeQuery()后，得到了一个ResultSet结构rs，rs包含SQL查询返回的所有行和一个指针，指针指向结果中第一行之前的位置，需要执行一次rs.next()才能让rs的指针指向第一个结果，同时返回true，之后每次执行rs.next()都会把指针移到下一个结果上并返回true，直至再也没有结果时，rs.next()的返回值变成了false。
还有一个细节，在执行建库建表、INSERT、UPDATE时，需要用executeUpdate()；在执行INSERT时，需要使用executeQuery()。executeQuery()总是返回一个ResultSet，executeUpdate()返回符合查询的行数。
httpGet.java的getByString()类负责向所给的网址发送请求，然后下载网页内容。
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HttpGet httpget = new
HttpGet(url);
System.out.println("executing request "
+ httpget.getURI());
ResponseHandlernew
ResponseHandler
public
String handleResponse(
final
HttpResponse response) throws
ClientProtocolException, IOException {
int
status = response.getStatusLine().getStatusCode();
if
(status >= 200
&& status < 300) {
HttpEntity entity = response.getEntity();
return
entity != null
? EntityUtils.toString(entity) : null;
} else
{
throw
new
ClientProtocolException("Unexpected response status: "
+ status);
}
}
};
String responseBody = httpclient.execute(httpget, responseHandler);
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这段代码是HTTPComponents的HTTP Client组件中给出的样例，在很多情况下可以直接使用。这部分代码获得了一个字符串responseBody，里面保存着网页中的全部字符。
接着，就需要把responseBody传递给parsePage.java的parseFromString类提取链接。
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Parser parser = new
Parser(content);
HasAttributeFilter filter = new
HasAttributeFilter("href");
try
{
NodeList list = parser.parse(filter);
int
count = list.size();
//process every link on this page
for(int
i=0; i
Node node = list.elementAt(i);
if(node instanceof
LinkTag) {
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在HTML文件中，链接一般都在a标签的href属性中，所以需要创建一个属性过滤器。NodeList保存着这个HTML文件中的所有DOM节点，通过在for循环中依次处理每个节点寻找符合要求的标签，可以把网页中的所有链接提取出来。
然后通过nextlink.startsWith()进一步筛选，只处理以“http://johnhany.net/”开头的链接并跳过以“http://johnhany.net/wp-content/”开头的链接。
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sql = "SELECT * FROM record WHERE URL = '"
+ nextlink + "'";
stmt = conn.createStatement(ResultSet.TYPE_FORWARD_ONLY,ResultSet.CONCUR_UPDATABLE);
rs = stmt.executeQuery(sql);
if(rs.next()) {
}else
{
//if the link does not exist in the database, insert it
sql = "INSERT INTO record (URL, crawled) VALUES ('"
+ nextlink + "',0)";
pstmt = conn.prepareStatement(sql, Statement.RETURN_GENERATED_KEYS);
pstmt.execute();
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在表record中查找是否已经存在这个链接，如果存在（rs.next()==true），不做任何处理；如果不存在（rs.next()==false），在表中插入这个地址并把crawled置为false。因为之前recordID设为AUTO_INCREMENT，所以要用 Statement.RETURN_GENERATED_KEYS获取适当的编号。
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nextlink = nextlink.substring(mainurl.length());
if(nextlink.startsWith("tag/")) {
tag = nextlink.substring(4, nextlink.length()-1);
tag = URLDecoder.decode(tag,"UTF-8");
sql = "INSERT INTO tags (tagname) VALUES ('"
+ tag + "')";
pstmt = conn.prepareStatement(sql, Statement.RETURN_GENERATED_KEYS);
pstmt.execute();
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去掉链接开头的“http://johnhany.net/”几个字符，提高字符比较的速度。如果含有“tag/”说明其后的字符是一个标签的名字，把这给名字提取出来，用UTF-8编码，保证汉字的正常显示，然后存入表tags。类似地还可以加入判断“article/”，“author/”，或“2013/11/”等对其他链接进行归类。
结果
这是两张数据库的截图，显示了程序的部分结果：
在这里可以获得全部输出结果。可以与博客的sitemap比较一下，看看如果想在其基础上实现sitemap生成工具，还要做哪些修改。
转: http://johnhany.net/2013/11/web-crawler-using-java-and-mysql/