java.util.Properties 繼承了 java.util.Hashtable<Object, Object>類
基本用法是new Properties()再調(diào)用load方法讀取相應(yīng)的配置文件

package com.demon.test.testProperties;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.Properties;

public class Demo {

    public static void main(String[] args) throws FileNotFoundException, IOException {
        Properties properties = new Properties();
//      properties.load(inStream);  //調(diào)用load(InputStream inStream);//從流中讀入配置文件
//      properties.load(reader);    //調(diào)用load(Reader reader);//從流中讀入配置文件
//      properties.loadFromXML(in); //調(diào)用loadFromXML(InputStream in)//從流中讀入xml格式配置文件
//      properties.store(out, comments);    //調(diào)用store(OutputStream out, String comments)//輸出配置文件到流
        
        properties.load(new FileInputStream("properties_InputStream"));
        properties.store(System.out, "show this properties  1");
        
        properties.load(new  FileReader("properties_Reader"));
        properties.store(System.out, "show this properties  2");
        
        properties.loadFromXML(new FileInputStream("XML_properties_InputStream"));
        properties.store(System.out, "show this properties  3");
    }
}

輸出如下：

#show this properties  1
#Mon Dec 25 17:52:12 CST 2017
name=demon
#show this properties  2
#Mon Dec 25 17:52:12 CST 2017
age=123
name=demon
#show this properties  3
#Mon Dec 25 17:52:12 CST 2017
age=123
name=demon.li
key=value

讀入的三個(gè)配置文件內(nèi)容為：
properties_InputStream：

name = demon

properties_Reader

name = demon
age = 123

XML_properties_InputStream

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">
<properties>
    <entry key="name">demon.li</entry>
    <entry key="key">value</entry>
</properties>

1.輸入

Properties 從流中讀入配置時(shí)調(diào)用的三個(gè)方法：

    public synchronized void load(Reader reader) throws IOException {
        load0(new LineReader(reader));
    }
    public synchronized void load(InputStream inStream) throws IOException {
        load0(new LineReader(inStream));
    }
    public synchronized void loadFromXML(InputStream in)
        throws IOException, InvalidPropertiesFormatException
    {
        XmlSupport.load(this, Objects.requireNonNull(in));
        in.close();
    }

可以看到，load(Reader reader)、load(InputStream inStream)是為load0 (LineReader lr)方法做了代理，分析load(InputStream inStream)方法和Properties$LineReader內(nèi)部類，可以看到，load0 (LineReader lr)調(diào)用了lr.readLine()方法從流中解析出一行有效的配置，再解析成對(duì)應(yīng)的key和value，調(diào)用Object java.util.Hashtable.put(Object key, Object value)將解析出來的key-value添加到自身(Hashtable)中

//java.util.Properties.load0(LineReader)
String key = loadConvert(lr.lineBuf, 0, keyLen, convtBuf);
String value = loadConvert(lr.lineBuf, valueStart, limit - valueStart, convtBuf);
put(key, value);

loadFromXML(InputStream in)方法調(diào)用了java.util.Properties.XmlSupport.load(Properties, InputStream)最終調(diào)用的是sun.util.spi.XmlPropertiesProvider.load(Properties arg0, InputStream arg1)方法，相對(duì)復(fù)雜但是從使用效果上看，和load是保持一致的，最后還是會(huì)將解析出來的key-value添加到自身去。
這種設(shè)計(jì)可以同時(shí)load多個(gè)配置文件，先load的配置有可能被覆蓋
要修改或增加配置，最好調(diào)用java.util.Properties.setProperty(String, String)方法，即使它只是做了代理

    public synchronized Object setProperty(String key, String value) {
        return put(key, value);
    }

2.輸出

Properties提供了list、save、store一系列方法幫助我們將配置寫入輸出流中，具體方法請(qǐng)alt+/
其中java.util.Properties.save(OutputStream, String)方法已廢棄，可以看到它代理了java.util.Properties.store(OutputStream, String)方法，我們使用時(shí)直接調(diào)用store系列方法即可
java.util.Properties.store(OutputStream, String)和java.util.Properties.store(Writer, String)方法是java.util.Properties.store0(BufferedWriter, String, boolean)方法的代理，從源碼可以看出該方法遍歷打印了所有的key-value，list方法也大同小異

    private void store0(BufferedWriter bw, String comments, boolean escUnicode)
        throws IOException
    {
        if (comments != null) {
            writeComments(bw, comments);
        }
        bw.write("#" + new Date().toString());
        bw.newLine();
        synchronized (this) {
            for (Enumeration<?> e = keys(); e.hasMoreElements();) {
                String key = (String)e.nextElement();
                String val = (String)get(key);
                key = saveConvert(key, true, escUnicode);
                /* No need to escape embedded and trailing spaces for value, hence
                 * pass false to flag.
                 */
                val = saveConvert(val, false, escUnicode);
                bw.write(key + "=" + val);
                bw.newLine();
            }
        }
        bw.flush();
    }

java.util.Properties.storeToXML(OutputStream, String)、java.util.Properties.storeToXML(OutputStream, String, String)這兩個(gè)方法和java.util.Properties.loadFromXML(InputStream)方法一樣，最后最為調(diào)用的是sun.util.spi.XmlPropertiesProvider.store(Properties arg0, OutputStream arg1, String arg2, String arg3)方法，效果和java.util.Properties.store(OutputStream, String)和java.util.Properties.store(Writer, String)方法保持一致。

兩個(gè)list方法和兩個(gè)store方法類似。

補(bǔ)充：需要注意，properties.load進(jìn)去后，輸入流不會(huì)被關(guān)閉，在讀取完配置文件后，應(yīng)該手動(dòng)關(guān)閉掉輸入流