20个非常有用的Java程序片段
1. 字符串有整型的相互转换
String a = String.valueOf(2); //integer to numeric string int i = Integer.parseInt(a); //numeric string to an int
2. 向文件末尾添加内容
BufferedWriter out = null;
try {
out = new BufferedWriter(new FileWriter(”filename”, true));
out.write(”aString”);
} catch (IOException e) {
// error processing code
} finally {
if (out != null) {
out.close();
}
}
3. 得到当前方法的名字
String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
4. 转字符串到日期
java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);
或者是：
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" ); Date date = format.parse( myString );
5. 使用JDBC链接Oracle
public class OracleJdbcTest
{
String driverClass = "oracle.jdbc.driver.OracleDriver";
Connection con;
public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException
{
Properties props = new Properties();
props.load(fs);
String url = props.getProperty("db.url");
String userName = props.getProperty("db.user");
String password = props.getProperty("db.password");
Class.forName(driverClass);
con=DriverManager.getConnection(url, userName, password);
}
public void fetch() throws SQLException, IOException
{
PreparedStatement ps = con.prepareStatement("select SYSDATE from dual");
ResultSet rs = ps.executeQuery();
while (rs.next())
{
// do the thing you do
}
rs.close();
ps.close();
}
public static void main(String[] args)
{
OracleJdbcTest test = new OracleJdbcTest();
test.init();
test.fetch();
}
}
6. 把 Java util.Date 转成 sql.Date
java.util.Date utilDate = new java.util.Date(); java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());
7. 使用NIO进行快速的文件拷贝
public static void fileCopy( File in, File out )
throws IOException
{
FileChannel inChannel = new FileInputStream( in ).getChannel();
FileChannel outChannel = new FileOutputStream( out ).getChannel();
try
{
// inChannel.transferTo(0, inChannel.size(), outChannel); // original -- apparently has trouble copying large files on Windows
// magic number for Windows, 64Mb - 32Kb)
int maxCount = (64 * 1024 * 1024) - (32 * 1024);
long size = inChannel.size();
long position = 0;
while ( position < size )
{
position += inChannel.transferTo( position, maxCount, outChannel );
}
}
finally
{
if ( inChannel != null )
{
inChannel.close();
}
if ( outChannel != null )
{
outChannel.close();
}
}
}
8. 创建图片的缩略图
private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename)
throws InterruptedException, FileNotFoundException, IOException
{
// load image from filename
Image image = Toolkit.getDefaultToolkit().getImage(filename);
MediaTracker mediaTracker = new MediaTracker(new Container());
mediaTracker.addImage(image, 0);
mediaTracker.waitForID(0);
// use this to test for errors at this point: System.out.println(mediaTracker.isErrorAny());
// determine thumbnail size from WIDTH and HEIGHT
double thumbRatio = (double)thumbWidth / (double)thumbHeight;
int imageWidth = image.getWidth(null);
int imageHeight = image.getHeight(null);
double imageRatio = (double)imageWidth / (double)imageHeight;
if (thumbRatio < imageRatio) {
thumbHeight = (int)(thumbWidth / imageRatio);
} else {
thumbWidth = (int)(thumbHeight * imageRatio);
}
// draw original image to thumbnail image object and
// scale it to the new size on-the-fly
BufferedImage thumbImage = new BufferedImage(thumbWidth, thumbHeight, BufferedImage.TYPE_INT_RGB);
Graphics2D graphics2D = thumbImage.createGraphics();
graphics2D.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
graphics2D.drawImage(image, 0, 0, thumbWidth, thumbHeight, null);
// save thumbnail image to outFilename
BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(outFilename));
JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
JPEGEncodeParam param = encoder.getDefaultJPEGEncodeParam(thumbImage);
quality = Math.max(0, Math.min(quality, 100));
param.setQuality((float)quality / 100.0f, false);
encoder.setJPEGEncodeParam(param);
encoder.encode(thumbImage);
out.close();
}
9. 创建 JSON 格式的数据
请先阅读这篇文章 了解一些细节，
并下面这个JAR 文件：json-rpc-1.0.jar (75 kb)
import org.json.JSONObject;
...
...
JSONObject json = new JSONObject();
json.put("city", "Mumbai");
json.put("country", "India");
...
String output = json.toString();
...
10. 使用iText JAR生成PDF
阅读这篇文章 了解更多细节
import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.util.Date;
import com.lowagie.text.Document;
import com.lowagie.text.Paragraph;
import com.lowagie.text.pdf.PdfWriter;
public class GeneratePDF {
public static void main(String[] args) {
try {
OutputStream file = new FileOutputStream(new File("C:\\Test.pdf"));
Document document = new Document();
PdfWriter.getInstance(document, file);
document.open();
document.add(new Paragraph("Hello Kiran"));
document.add(new Paragraph(new Date().toString()));
document.close();
file.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
11. HTTP 代理设置
阅读这篇 文章 了解更多细节。
System.getProperties().put("http.proxyHost", "someProxyURL");
System.getProperties().put("http.proxyPort", "someProxyPort");
System.getProperties().put("http.proxyUser", "someUserName");
System.getProperties().put("http.proxyPassword", "somePassword");
12. 单实例Singleton 示例
请先阅读这篇文章 了解更多信息
public class SimpleSingleton {
private static SimpleSingleton singleInstance = new SimpleSingleton();
//Marking default constructor private
//to avoid direct instantiation.
private SimpleSingleton() {
}
//Get instance for class SimpleSingleton
public static SimpleSingleton getInstance() {
return singleInstance;
}
}
另一种实现
public enum SimpleSingleton {
INSTANCE;
public void doSomething() {
}
}
//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();
13. 抓屏程序
阅读这篇文章 获得更多信息。
import java.awt.Dimension;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.Toolkit;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import java.io.File;
...
public void captureScreen(String fileName) throws Exception {
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
Rectangle screenRectangle = new Rectangle(screenSize);
Robot robot = new Robot();
BufferedImage image = robot.createScreenCapture(screenRectangle);
ImageIO.write(image, "png", new File(fileName));
}
...
14. 列出文件和目录
File dir = new File("directoryName");
String[] children = dir.list();
if (children == null) {
// Either dir does not exist or is not a directory
} else {
for (int i=0; i < children.length; i++) {
// Get filename of file or directory
String filename = children[i];
}
}
// It is also possible to filter the list of returned files.
// This example does not return any files that start with `.'.
FilenameFilter filter = new FilenameFilter() {
public boolean accept(File dir, String name) {
return !name.startsWith(".");
}
};
children = dir.list(filter);
// The list of files can also be retrieved as File objects
File[] files = dir.listFiles();
// This filter only returns directories
FileFilter fileFilter = new FileFilter() {
public boolean accept(File file) {
return file.isDirectory();
}
};
files = dir.listFiles(fileFilter);
15. 创建ZIP和JAR文件
import java.util.zip.*;
import java.io.*;
public class ZipIt {
public static void main(String args[]) throws IOException {
if (args.length < 2) {
System.err.println("usage: java ZipIt Zip.zip file1 file2 file3");
System.exit(-1);
}
File zipFile = new File(args[0]);
if (zipFile.exists()) {
System.err.println("Zip file already exists, please try another");
System.exit(-2);
}
FileOutputStream fos = new FileOutputStream(zipFile);
ZipOutputStream zos = new ZipOutputStream(fos);
int bytesRead;
byte[] buffer = new byte[1024];
CRC32 crc = new CRC32();
for (int i=1, n=args.length; i < n; i++) {
String name = args[i];
File file = new File(name);
if (!file.exists()) {
System.err.println("Skipping: " + name);
continue;
}
BufferedInputStream bis = new BufferedInputStream(
new FileInputStream(file));
crc.reset();
while ((bytesRead = bis.read(buffer)) != -1) {
crc.update(buffer, 0, bytesRead);
}
bis.close();
// Reset to beginning of input stream
bis = new BufferedInputStream(
new FileInputStream(file));
ZipEntry entry = new ZipEntry(name);
entry.setMethod(ZipEntry.STORED);
entry.setCompressedSize(file.length());
entry.setSize(file.length());
entry.setCrc(crc.getValue());
zos.putNextEntry(entry);
while ((bytesRead = bis.read(buffer)) != -1) {
zos.write(buffer, 0, bytesRead);
}
bis.close();
}
zos.close();
}
}
16. 解析/读取XML 文件
XML文件
<?xml version="1.0"?>
<students>
<student>
<name>John</name>
<grade>B</grade>
<age>12</age>
</student>
<student>
<name>Mary</name>
<grade>A</grade>
<age>11</age>
</student>
<student>
<name>Simon</name>
<grade>A</grade>
<age>18</age>
</student>
</students>
Java代码
package net.viralpatel.java.xmlparser;
import java.io.File;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
public class XMLParser {
public void getAllUserNames(String fileName) {
try {
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
File file = new File(fileName);
if (file.exists()) {
Document doc = db.parse(file);
Element docEle = doc.getDocumentElement();
// Print root element of the document
System.out.println("Root element of the document: "
+ docEle.getNodeName());
NodeList studentList = docEle.getElementsByTagName("student");
// Print total student elements in document
System.out
.println("Total students: " + studentList.getLength());
if (studentList != null && studentList.getLength() > 0) {
for (int i = 0; i < studentList.getLength(); i++) {
Node node = studentList.item(i);
if (node.getNodeType() == Node.ELEMENT_NODE) {
System.out
.println("=====================");
Element e = (Element) node;
NodeList nodeList = e.getElementsByTagName("name");
System.out.println("Name: "
+ nodeList.item(0).getChildNodes().item(0)
.getNodeValue());
nodeList = e.getElementsByTagName("grade");
System.out.println("Grade: "
+ nodeList.item(0).getChildNodes().item(0)
.getNodeValue());
nodeList = e.getElementsByTagName("age");
System.out.println("Age: "
+ nodeList.item(0).getChildNodes().item(0)
.getNodeValue());
}
}
} else {
System.exit(1);
}
}
} catch (Exception e) {
System.out.println(e);
}
}
public static void main(String[] args) {
XMLParser parser = new XMLParser();
parser.getAllUserNames("c:\\test.xml");
}
}
17. 把 Array 转换成 Map
import java.util.Map;
import org.apache.commons.lang.ArrayUtils;
public class Main {
public static void main(String[] args) {
String[][] countries = { { "United States", "New York" }, { "United Kingdom", "London" },
{ "Netherland", "Amsterdam" }, { "Japan", "Tokyo" }, { "France", "Paris" } };
Map countryCapitals = ArrayUtils.toMap(countries);
System.out.println("Capital of Japan is " + countryCapitals.get("Japan"));
System.out.println("Capital of France is " + countryCapitals.get("France"));
}
}
18. 发送邮件
import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;
public void postMail( String recipients[ ], String subject, String message , String from) throws MessagingException
{
boolean debug = false;
//Set the host smtp address
Properties props = new Properties();
props.put("mail.smtp.host", "smtp.example.com");
// create some properties and get the default Session
Session session = Session.getDefaultInstance(props, null);
session.setDebug(debug);
// create a message
Message msg = new MimeMessage(session);
// set the from and to address
InternetAddress addressFrom = new InternetAddress(from);
msg.setFrom(addressFrom);
InternetAddress[] addressTo = new InternetAddress[recipients.length];
for (int i = 0; i < recipients.length; i++)
{
addressTo[i] = new InternetAddress(recipients[i]);
}
msg.setRecipients(Message.RecipientType.TO, addressTo);
// Optional : You can also set your custom headers in the Email if you Want
msg.addHeader("MyHeaderName", "myHeaderValue");
// Setting the Subject and Content Type
msg.setSubject(subject);
msg.setContent(message, "text/plain");
Transport.send(msg);
}
19. 发送代数据的HTTP 请求
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;
public class Main {
public static void main(String[] args) {
try {
URL my_url = new URL("http://coolshell.cn/");
BufferedReader br = new BufferedReader(new InputStreamReader(my_url.openStream()));
String strTemp = "";
while(null != (strTemp = br.readLine())){
System.out.println(strTemp);
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
20. 改变数组的大小
/**
* Reallocates an array with a new size, and copies the contents
* of the old array to the new array.
* @param oldArray the old array, to be reallocated.
* @param newSize the new array size.
* @return A new array with the same contents.
*/
private static Object resizeArray (Object oldArray, int newSize) {
int oldSize = java.lang.reflect.Array.getLength(oldArray);
Class elementType = oldArray.getClass().getComponentType();
Object newArray = java.lang.reflect.Array.newInstance(
elementType,newSize);
int preserveLength = Math.min(oldSize,newSize);
if (preserveLength > 0)
System.arraycopy (oldArray,0,newArray,0,preserveLength);
return newArray;
}
// Test routine for resizeArray().
public static void main (String[] args) {
int[] a = {1,2,3};
a = (int[])resizeArray(a,5);
a[3] = 4;
a[4] = 5;
for (int i=0; i<a.length; i++)
System.out.println (a[i]);
}
21.java操作excel
工具类：
package com.xxxxx.cnnvd.util;
import java.io.File; import java.io.FileInputStream; import java.io.IOException; import java.util.ArrayList; import java.util.Iterator;
import org.apache.poi.hssf.usermodel.HSSFCell; import org.apache.poi.hssf.usermodel.HSSFRow; import org.apache.poi.hssf.usermodel.HSSFSheet; import org.apache.poi.hssf.usermodel.HSSFWorkbook; import org.apache.poi.poifs.filesystem.POIFSFileSystem; import org.apache.poi.xssf.usermodel.XSSFCell; import org.apache.poi.xssf.usermodel.XSSFRow; import org.apache.poi.xssf.usermodel.XSSFSheet; import org.apache.poi.xssf.usermodel.XSSFWorkbook;
public class ReadExcelTool { public ArrayList<ArrayList<String>> readExcel(String fileName,String path) { ArrayList<ArrayList<String>> Row =new ArrayList<ArrayList<String>>(); try { FileInputStream fileIn = new FileInputStream(new File(path,fileName)); POIFSFileSystem fs = new POIFSFileSystem(fileIn); HSSFWorkbook wb = new HSSFWorkbook(fs); for (int numSheet = 0; numSheet < wb.getNumberOfSheets(); numSheet++) { HSSFSheet xssfSheet = wb.getSheetAt(numSheet); if (xssfSheet == null) { continue; } // 循环行Row for (int rowNum = 1; rowNum <= xssfSheet.getLastRowNum(); rowNum++) { HSSFRow xssfRow = xssfSheet.getRow(rowNum); if (xssfRow == null) { continue; } // 循环列Cell ArrayList<String> cell =new ArrayList<String>(); for (int cellNum = 0; cellNum <= xssfRow.getLastCellNum(); cellNum++) { HSSFCell xssfCell = xssfRow.getCell(cellNum); if (xssfCell == null) { continue; } cell.add(getValue(xssfCell)); } Row.add(cell); } } } catch (IOException e) { e.printStackTrace(); } return Row; }
private String getValue(HSSFCell xssfCell) { if (xssfCell.getCellType() == xssfCell.CELL_TYPE_BOOLEAN) { return String.valueOf(xssfCell.getBooleanCellValue()); } else if (xssfCell.getCellType() == xssfCell.CELL_TYPE_NUMERIC) { return String.valueOf(xssfCell.getNumericCellValue()); } else { return String.valueOf(xssfCell.getStringCellValue()); } } /**
//测试类 public static void main(String[] args) { ReadExcelTool ret = new ReadExcelTool(); ArrayList<ArrayList<String>> datas = ret.readExcel("template.xls", "D:\\java\\tomcat60\\webapps\\zyst\\impfiles"); for (Iterator iterator = datas.iterator(); iterator.hasNext();) { ArrayList<String> datas2 = (ArrayList<String>) iterator.next(); for (Iterator iterator2 = datas2.iterator(); iterator2.hasNext();) { String name = (String) iterator2.next(); System.out.print(name); } System.out.println(); } } **/ }
使用工具类
/** * 检查文件的后缀名称是否xls * * @param fileFileName2 * @return 返回文件后缀名xls,若不符合则返回null */ private String checkSuffix(String fileFileName2) { if (fileFileName2 == null) { return null; } String fileName = fileFileName2.substring(fileFileName2.lastIndexOf(".") + 1); if (fileName.equals("xls")) { return "xls"; } return null; }
/** * 把excel数据导入数据库 * * @return */ public String cnnvdImportSave() { String path = sRequest.getSession().getServletContext().getRealPath("/uploadfiles"); // System.out.println(path); //必须是符合格式的excel文件 if ("xls".equals(checkSuffix(fileFileName))) { //路径 // ServletActionContext.getServletContext().getRealPath("/"); File savefile = new File(new File(path), fileFileName); if (!savefile.getParentFile().exists()) { savefile.getParentFile().mkdirs(); } try { FileUtils.copyFile(file, savefile); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } //读取excel文件 ReadExcelTool ret = new ReadExcelTool(); ArrayList<ArrayList<String>> datas = ret.readExcel(fileFileName, path); // SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd"); if (datas != null && datas.size() > 0) { for (int i = 0; i < datas.size(); i++) { List temp = datas.get(i); if (temp != null && temp.size() > 0) { String vulname = (String) temp.get(3); String levelid = (String) temp.get(30); String vultype = (String) temp.get(39); String vulpubtime = (String) temp.get(13); String vulreptime = (String) temp.get(14); Date vulpubtimeDate = null; Date vulreptimeDate = null; try { vulpubtimeDate = sdf.parse(vulpubtime); vulreptimeDate = sdf.parse(vulreptime); } catch (ParseException e) { // TODO Auto-generated catch block e.printStackTrace(); } String vulorg = (String) temp.get(40); String vulsummary = (String) temp.get(4); String pub = (String) temp.get(55); String refurl = (String) temp.get(19); String cnnvdid = (String) temp.get(1); String cveid = (String) temp.get(9); String bid = (String) temp.get(10); //通过cnnvdid判断该条数据是否已存在,存在就先删除,再新增 if (vulnerabilityService.checkCnnvdIdExist(cnnvdid) != null) { String sql4delete = "delete from t_cnnvd_vulnerability where cnnvdid = ?"; jdbcTemplate.update(sql4delete,cnnvdid); } String sql = "insert into t_cnnvd_xxxxxxx(vulname,levelid,vultype,vulpubtime,vulreptime,vulorg,vulsummary,pub,refurl,cnnvdid,cveid,bid) values(?,?,?,?,?,?,?,?,?,?,?,?)"; jdbcTemplate.update(sql,vulname,levelid,vultype,vulpubtime,vulreptime,vulorg,vulsummary,pub,refurl,cnnvdid,cveid,bid); } } } //把生成的临时文件删除 savefile.delete(); request.put("cnnvdImportResult", "success"); return SUCCESS; } return ERROR; }
22.java操作csv，使用CsvReader，需要下载使用javacsv.jar
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.List;
import org.apache.log4j.Logger;
import com.csvreader.CsvReader;
import com.csvreader.CsvWriter;
/**
* CSV测试类
*
*/
public class CSVTest {
protected static Logger logger = Logger.getLogger(CSVTest.class); /**
* 读取CSV文件
* @param file
* @return
*/
public static List<String[]> readCsv(String file) {
List<String[]> list = new ArrayList<String[]>();
CsvReader reader = null;
try {
// 初始化CsvReader并指定列分隔符和字符编码
reader = new CsvReader(file, ',', Charset.forName("GBK"));
while (reader.readRecord()) {
// 读取每行数据以数组形式返回
String[] str = reader.getValues();
if (str != null && str.length > 0) {
//reader.getCurrentRecord()得到当前记录的行号,第一行是头，不使用
if ((reader.getCurrentRecord() != 0) && (str != null) && (str.length > 0)) {
System.out.println(str.length);
System.out.println(str[1]);
list.add(str);
}
}
}
} catch (FileNotFoundException e) {
logger.error(e.getMessage(), e);
} catch (IOException e) {
logger.error(e.getMessage(), e);
} finally {
if (reader != null) {
reader.close();
}
}
return list;
}
/** * 写入CSV文件 */ public static void writeCsv(List<String[]> content, String path) { CsvWriter writer = null; try { writer = new CsvWriter(path, ',', Charset.forName("GBK")); if(content != null && content.size() != 0) { for(String[] strArray : content) { writer.writeRecord(strArray); } } else { // 做异常处理 } } catch (IOException e) { logger.error(e.getMessage(), e); } finally { if(writer != null) { writer.close(); } } }
/** * @param args * @throws Exception */ public static void main(String[] args) throws Exception { String readPath = "F:" + File.separator + "test" + File.separator + "muti-test.csv"; String writePath = "F:" + File.separator + "test" + File.separator + "muti-test-output.csv"; List<String[]> result = readCsv(readPath); for(String[] strArray : result) { for(String s : strArray) { System.out.print(s + ","); } System.out.println(); } writeCsv(result, writePath); Thread.sleep(999999); } }
