转自http://blog.csdn.net/bzwm/archive/2009/02/12/3881392.aspx
一、在研究join的用法之前，先明确两件事情。
1.join方法定义在Thread类中，则调用者必须是一个线程，
例如：
Thread t = new CustomThread();//这里一般是自定义的线程类
t.start();//线程起动
t.join();//此处会抛出InterruptedException异常
2.上面的两行代码也是在一个线程里面执行的。
以上出现了两个线程，一个是我们自定义的线程类，我们实现了run方法，做一些我们需要的工作；另外一个线程，生成我们自定义线程类的对象，然后执行
customThread.start();
customThread.join();
在这种情况下，两个线程的关系是一个线程由另外一个线程生成并起动，所以我们暂且认为第一个线程叫做“子线程”，另外一个线程叫做“主线程”。
二、为什么要用join()方法
主线程生成并起动了子线程，而子线程里要进行大量的耗时的运算(这里可以借鉴下线程的作用)，当主线程处理完其他的事务后，需要用到子线程的处理结果，这个时候就要用到join();方法了。
三、join方法的作用
在网上看到有人说“将两个线程合并”。这样解释我觉得理解起来还更麻烦。不如就借鉴下API里的说法：
“等待该线程终止。”
解释一下，是主线程(我在“一”里已经命名过了)等待子线程的终止。也就是在子线程调用了join()方法后面的代码，只有等到子线程结束了才能执行。(Waits for this thread to die.)
四、用实例来理解
写一个简单的例子来看一下join()的用法，一共三个类：
1.CustomThread 类
2. CustomThread1类
3. JoinTestDemo 类，main方法所在的类。
代码1：
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package wxhx.csdn2;
/**
*
* @author bzwm
*
*/
class CustomThread1 extends Thread {
public CustomThread1() {
super("[CustomThread1] Thread");
};
public void run() {
String threadName = Thread.currentThread().getName();
System.out.println(threadName + " start.");
try {
for (int i = 0; i < 5; i++) {
System.out.println(threadName + " loop at " + i);
Thread.sleep(1000);
}
System.out.println(threadName + " end.");
} catch (Exception e) {
System.out.println("Exception from " + threadName + ".run");
}
}
}
class CustomThread extends Thread {
CustomThread1 t1;
public CustomThread(CustomThread1 t1) {
super("[CustomThread] Thread");
this.t1 = t1;
}
public void run() {
String threadName = Thread.currentThread().getName();
System.out.println(threadName + " start.");
try {
t1.join();
System.out.println(threadName + " end.");
} catch (Exception e) {
System.out.println("Exception from " + threadName + ".run");
}
}
}
public class JoinTestDemo {
public static void main(String[] args) {
String threadName = Thread.currentThread().getName();
System.out.println(threadName + " start.");
CustomThread1 t1 = new CustomThread1();
CustomThread t = new CustomThread(t1);
try {
t1.start();
Thread.sleep(2000);
t.start();
t.join();//在代碼2里，將此處注釋掉
} catch (Exception e) {
System.out.println("Exception from main");
}
System.out.println(threadName + " end!");
}
}
打印结果：
main start.//main方法所在的线程起动，但没有马上结束，因为调用t.join();，所以要等到t结束了，此线程才能向下执行。
[CustomThread1] Thread start.//线程CustomThread1起动
[CustomThread1] Thread loop at 0//线程CustomThread1执行
[CustomThread1] Thread loop at 1//线程CustomThread1执行
[CustomThread] Thread start.//线程CustomThread起动，但没有马上结束，因为调用t1.join();，所以要等到t1结束了，此线程才能向下执行。
[CustomThread1] Thread loop at 2//线程CustomThread1继续执行
[CustomThread1] Thread loop at 3//线程CustomThread1继续执行
[CustomThread1] Thread loop at 4//线程CustomThread1继续执行
[CustomThread1] Thread end. //线程CustomThread1结束了
[CustomThread] Thread end.// 线程CustomThread在t1.join();阻塞处起动，向下继续执行的结果
main end!//线程CustomThread结束，此线程在t.join();阻塞处起动，向下继续执行的结果。
修改一下代码，得到代码2：(这里只写出修改的部分)
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public class JoinTestDemo {
public static void main(String[] args) {
String threadName = Thread.currentThread().getName();
System.out.println(threadName + " start.");
CustomThread1 t1 = new CustomThread1();
CustomThread t = new CustomThread(t1);
try {
t1.start();
Thread.sleep(2000);
t.start();
// t.join();//在代碼2里，將此處注釋掉
} catch (Exception e) {
System.out.println("Exception from main");
}
System.out.println(threadName + " end!");
}
}
打印结果：
main start. // main方法所在的线程起动，但没有马上结束，这里并不是因为join方法，而是因为Thread.sleep(2000);
[CustomThread1] Thread start. //线程CustomThread1起动
[CustomThread1] Thread loop at 0//线程CustomThread1执行
[CustomThread1] Thread loop at 1//线程CustomThread1执行
main end!// Thread.sleep(2000);结束，虽然在线程CustomThread执行了t1.join();，但这并不会影响到其他线程(这里main方法所在的线程)。
[CustomThread] Thread start. //线程CustomThread起动，但没有马上结束，因为调用t1.join();，所以要等到t1结束了，此线程才能向下执行。
[CustomThread1] Thread loop at 2//线程CustomThread1继续执行
[CustomThread1] Thread loop at 3//线程CustomThread1继续执行
[CustomThread1] Thread loop at 4//线程CustomThread1继续执行
[CustomThread1] Thread end. //线程CustomThread1结束了
[CustomThread] Thread end. // 线程CustomThread在t1.join();阻塞处起动，向下继续执行的结果
五、从源码看join()方法
在CustomThread的run方法里，执行了t1.join();，进入看一下它的JDK源码：
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public final void join() throws InterruptedException {
n(0);
}
然后进入join(0)方法：
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/**
* Waits at most <code>millis</code> milliseconds for this thread to
* die. A timeout of <code>0</code> means to wait forever. //注意这句
*
* @param millis the time to wait in milliseconds.
* @exception InterruptedException if another thread has interrupted
* the current thread. The <i>interrupted status</i> of the
* current thread is cleared when this exception is thrown.
*/
public final synchronized void join(long millis) //参数millis为0.
throws InterruptedException {
long base = System.currentTimeMillis();
long now = 0;
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (millis == 0) {//进入这个分支
while (isAlive()) {//判断本线程是否为活动的。这里的本线程就是t1.
wait(0);//阻塞
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
单纯从代码上看，如果线程被生成了，但还未被起动，调用它的join()方法是没有作用的。将直接继续向下执行，这里就不写代码验证了。
listView = new ListView(this);
prepareData();
//适配器提供 data到layout的接口
SimpleAdapter simpleAdapter = new SimpleAdapter(this, data, android.R.layout.simple_list_item_2,
new String[]{"性别","姓名"}, new int[]{R.id.text1,R.id.text2}); //只显示性别对应的数据
listView.setAdapter(simpleAdapter);
setContentView(listView);
listView.setOnItemClickListener(new OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {
//parent.getItemAtPosition(position)会得到对应这行的data
setTitle(parent.getItemAtPosition(position).toString()); //这里title将取到全部数据
}
});
//==============技巧，因为listView能展现出来，说明Cursor已经拿到了，所以取数据可以通过Cursot==========================
Cursor c = mTrackCursor;
c.moveToPosition(position);
Intent i = new Intent(this, ShowTrack.class);
i.putExtra(TrackDbAdapter.KEY_ROWID, id);
i.putExtra(TrackDbAdapter.NAME, c.getString(c
.getColumnIndexOrThrow(TrackDbAdapter.NAME)));
i.putExtra(TrackDbAdapter.DESC, c.getString(c
.getColumnIndexOrThrow(TrackDbAdapter.DESC)));
c.getString()为得到某个索引的值
Intent emailIntent = new Intent(Intent.ACTION_SEND);
emailIntent.setType("text/html");
emailIntent.putExtra(android.content.Intent.EXTRA_EMAIL,sendTo );
emailIntent.putExtra(android.content.Intent.EXTRA_SUBJECT, "test" );
emailIntent.putExtra(android.content.Intent.EXTRA_TEXT, "msg" );
2.必须使用外部java mail library 以及写一个 类似下面的helper class
public class GMailSender extends javax.mail.Authenticator {
private String mailhost = "smtp.gmail.com";
private String user;
private String password;
private Session session;
static {
Security.addProvider(new JSSEProvider());
}
public GMailSender(String user, String password) {
this.user = user;
this.password = password;
Properties props = new Properties();
props.setProperty("mail.transport.protocol", "smtp");
props.setProperty("mail.host", mailhost);
props.put("mail.smtp.auth", "true");
props.put("mail.smtp.port", "465");
props.put("mail.smtp.socketFactory.port", "465");
props.put("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory");
props.put("mail.smtp.socketFactory.fallback", "false");
props.setProperty("mail.smtp.quitwait", "false");
session = Session.getDefaultInstance(props, this);
}
protected PasswordAuthentication getPasswordAuthentication() {
return new PasswordAuthentication(user, password);
}
public synchronized void sendMail(String subject, String body, String sender, String recipients) throws Exception {
MimeMessage message = new MimeMessage(session);
DataHandler handler = new DataHandler(new ByteArrayDataSource(body.getBytes(), "text/plain"));
message.setSender(new InternetAddress(sender));
message.setSubject(subject);
message.setDataHandler(handler);
if (recipients.indexOf(',') > 0)
message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(recipients));
else
message.setRecipient(Message.RecipientType.TO, new InternetAddress(recipients));
Transport.send(message);
}
public class ByteArrayDataSource implements DataSource {
private byte[] data;
private String type;
public ByteArrayDataSource(byte[] data, String type) {
super();
this.data = data;
this.type = type;
}
public ByteArrayDataSource(byte[] data) {
super();
this.data = data;
}
public void setType(String type) {
this.type = type;
}
public String getContentType() {
if (type == null)
return "application/octet-stream";
else
return type;
}
public InputStream getInputStream() throws IOException {
return new ByteArrayInputStream(data);
}
public String getName() {
return "ByteArrayDataSource";
}
public OutputStream getOutputStream() throws IOException {
throw new IOException("Not Supported");
}
}
}
Sending the email is t