java 读写文件

用Apache POI读excel(.xlsx)文件

//新建输入字节流
FileInputStream input = new FileInputStream(file);
//获取XSSFWorkbook对象，代表工作簿
XSSFWorkbook book = new XSSFWorkbook(input);
//获取XSSFSheet对象，代表第一张工作表
XSSFSheet sheet = book.getSheetAt(0);
//遍历工作表的所有存在记录的行
Iterator<Row> rowIt = sheet.cellIterator();
Row row;
Cell cell;
//表示整个工作簿的内容
List<Object> content = new ArrayList<Object>();
//表示工作表的一行的记录
List<String> entry;
while(rowIt.hasNext()) {
    row = rowIt.next();
    Iterator<Cell> cellIt = row.iterator();
    entry = new ArrayList<String>();
    while(cellIt.hasNext()) {
        cell =  cellIt.next();
        //判断单元格类型
        switch(cell.getCellType()) {
            case STRING:
                entry.add(cell.getStringCellValue());
                break;
            case BLANK:
                break;
            default:
                break;
        }
    }
    content.add(entry);
}

用Apache POI写excel(.xlsx)文件

//创建工作簿
XSSFWorkbook book = new XSSFWorkbook();
//新增工作表
XSSFSheet sheet = book.createSheet();
XSSFRow row;
XSSFCell cell;

//entry collection
Map<Integer, Object[]> entries = new HashMap<Integer, Object[]>();
entries.put(0, new Object[]{"name", "sex"});
entries.put(1, new Object[]{"xzj", "male"});
entries.put(2, new Object[]{"luise", "female"});

Set<Integer> rownumSet = entries.keySet();
for(int rownum : rownumSet) {
    //新增一行
    row = sheet.createRow(rownum);
    Object[] entry = entries.get(rownum);
    int index = 0;
    for(Object param : entry) {
        //新增单元格
        cell = row.createCell(index++);
        //设置单元格的值
        cell.setCellValue(param.toString());
    }
}

FileOutputStream output = new FileOutputStream(file);
//把工作簿写入到字节流
book.write(output);

使用JAXP(Java API for XML Processing)读写XML文档，支持两种解析方式：DOM和SAX(Simple API for XML)。用DOM解析XML文档，需要读取整个文档。用SAX解析XML文档，一边读取文档一边解析。

解析DOM读XML文档

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
//创建DocumentBuilder对象
DocumentBuilder builder = factory.newDocumentBuilder();
//解析文件
Document doc = builder.parse(new File("/dir"));
//根据标签名称获取根节点
NodeList nodeList = doc.getElementsByTagName("tag_name");
//保存节点数据
List<String> content = new ArrayList<String>();
//子节点列表
NodeList subNodeList;
//子节点
Node subNode;
for(int i=0; i<nodeList.getLength(); i++) {
    //获取子节点
    subNodeList = nodeList.item(i).getChildNodes();
    for (int j=0; j<subNodeList.getLength(); j++) {
        //获取子节点的子节点
        subNode = subNodeList.item(j);
        //判断节点类型
        if(subNode.getNodeType() == Node.ELEMENT_NODE) {
            content.add(subNode.getTextContent());
        }
    }
}

构造DOM写XML文档

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
//创建DocumentBuilder对象
DocumentBuilder builder = factory.newDocumentBuilder();
//创建Document对象
Document doc = builder.newDocument();
//设置xml文档version
doc.setXmlVersion("1.0");
doc.setXmlStandalone(true);
//创建根节点
Element root = doc.createElement("info");
//创建person子节点
Element person = doc.createElement("person");
//创建name子节点
Element name = doc.createElement("name");
name.setTextContent("xzj");
//把name子节点添加到person节点
person.appendChild(name);
//创建sex子节点
Element sex = doc.createElement("sex");
sex.setTextContent("male");
//把sex子节点添加到person节点
person.appendChild(sex);
//把person节点添加到根节点
root.appendChild(person);
//把根节点添加到xml文档
doc.appendChild(root);
TransformerFactory transFormers = TransformerFactory.newInstance();
Transformer transFormer = transFormers.newTransformer();
//设置缩进
transFormer.setOutputProperty(OutputKeys.INDENT, "yes");
//把Document对象所代表的xml文档写入xml文件
transFormer.transform(new DOMSource(doc), new StreamResult(new File("/dir")));

用dom4j读XML文档

SAXReader reader = new SAXReader();
//读取文件
Document doc = reader.read(new File("/dir"));
//获取根节点
Element root = doc.getRootElement();
//获取根节点包含的子节点
List<Element> elements = root.elements();
//保存所有节点的数据
List<Object> content = new ArrayList<Object>();
//保存一个节点的数据
List<String> tag = new ArrayList<String>();
for(Element e : elements) {
    //根据标签名称获取标签内容
    tag.add(e.element("tag_name").getText());
    content.add(tag);
    tag = new ArrayList<String>();
}

用dom4j写XML文档

//新建Document对象
Document doc = DocumentHelper.createDocument();
//增加根节点
Element root = doc.addElement("info");
//增加person子节点
Element person = root.addElement("person");
//增加name子节点
Element name = person.addElement("name").addText("xzj");
//增加sex子节点
Element sex = person.addElement("sex").addText("male");
FileWriter out = new FileWriter("/dir");
XMLWriter writer = new XMLWriter(out);
writer.write(doc);
writer.close();

从数据库中读取图片

public static final int LENGTH = 20000;
String fileName = "image";
String path = "E:\\images\\";
byte[] b = new byte[LENGTH];
File file = null;
int total;      
while(resultSet.next()){
    fileName = rs.getString("bid");
    InputStream input = rs.getBinaryStream("filebody");
    if(input == null) {
        continue;
    }
    input.read(b);
    file = new File(path+fileName+".jpg");
    OutputStream output = new FileOutputStream(file);
    output.write(b);
    System.out.printf("图片有%dbyte\n", total);
}

读取properties文件

1. 使用java.util.ResourceBundle
2. 使用java.util.Properties
   路径：src/main/resources/demo.properties
   代码：

Properties properties = new Properties();
InputStream stream = this.getClass().getResourceAsStream("/demo.properties");
properties.load(stream);
String value = properties.getProperty("key");
Map<String, String> hashMap = new HashMap<String, String>();
hashMap.put("key", value);