Android Thread 介紹與執行個體

Android中很重要的一個機制就是線程+訊息，當然線程並不是android專屬的，下面，簡單的說說使用線程的時候應該注意的地方

我們採用最簡單的方法來建立一個android的線程+訊息的例子

1.Thread + Handler

[java]

複製代碼 代碼如下:package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

TextView mTextView = null;
// static TextView mTextView = null;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTextView = (TextView)findViewById(R.id.textview);
Thread th = new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
for(int i = 0;i<1000;i++)
{
try {
Thread.sleep(500);
System.out.println("Thread running :"+i+"!");
Message msg = new Message();
msg.what = i;
mHandler.sendMessage(msg);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
});
th.start();

}
public Handler mHandler = new Handler(){
// public static Handler mHandler = new Handler(){

@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
super.handleMessage(msg);

mTextView.setText(String.valueOf(msg.what));
}

};

}

package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

TextView mTextView = null;
// static TextView mTextView = null;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTextView = (TextView)findViewById(R.id.textview);
Thread th = new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
for(int i = 0;i<1000;i++)
{
try {
Thread.sleep(500);
System.out.println("Thread running :"+i+"!");
Message msg = new Message();
msg.what = i;
mHandler.sendMessage(msg);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
});
th.start();

}
public Handler mHandler = new Handler(){
// public static Handler mHandler = new Handler(){

@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
super.handleMessage(msg);

mTextView.setText(String.valueOf(msg.what));
}

};

}

當我們用以上方式建立線程時，進入應用之後，線程開始運行，Handler接收訊息改變UI中的TextView，此時一切正常

當按下退出時，程式退出，但是程式進程還在stack中，因此主線程之子線程，也就是我們定義的th（th_1）不會退出，此時，在log資訊中可以看到，system.out還在print數字

當再次進入程式的時候，可以看到，log中列印的資訊double，但是UI會按照新線程（th_2）的次序改變

此時th_1仍在運行，th_1使用的 handler_1也在運行，只不過上一個Activity的狀態已經是finish，因此不會改變UI this ->mFinished= true

其實只要th_1中有關於上一個Activity的引用，那麼Activity就不會銷毀，java的機制就是這樣，這是我們推薦的線程機制，下面著重說一下可能遇到的問題

2.同樣是剛剛的例子，我們將Handler定義成static

[java]
public static Handler mHandler = new Handler(){

public static Handler mHandler = new Handler(){此時，在退出應用再重新進入時，由於Handler並不會有新的執行個體，因此，th_1與th_2同時發訊息給一個static Handler 或者說是指向了同一塊記憶體地區，這時就會出現TextView上的數字來回跳的現象

3.這樣也可以

使用static定義Handler也不是不可以，只要在Activity的onCreate()中重新執行個體一個Handler，這樣，JVM分配另一塊記憶體給新的Handler，這樣運行就正常了

[java]

複製代碼 代碼如下:package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

public Handler mHandler = null;
TextView mTextView = null;
// static TextView mTextView = null;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTextView = (TextView)findViewById(R.id.textview);
mHandler = new TestHandler();
Thread th = new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
for(int i = 0;i<1000;i++)
{
try {
Thread.sleep(500);
System.out.println("Thread running :"+i+"!");
Message msg = new Message();
msg.what = i;
mHandler.sendMessage(msg);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
});
th.start();

}
class TestHandler extends Handler
{
@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
super.handleMessage(msg);
System.out.println("Handler running :"+msg.what+"!");
mTextView.setText(String.valueOf(msg.what));
}

}
}

package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

public Handler mHandler = null;
TextView mTextView = null;
// static TextView mTextView = null;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTextView = (TextView)findViewById(R.id.textview);
mHandler = new TestHandler();
Thread th = new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
for(int i = 0;i<1000;i++)
{
try {
Thread.sleep(500);
System.out.println("Thread running :"+i+"!");
Message msg = new Message();
msg.what = i;
mHandler.sendMessage(msg);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
});
th.start();

}
class TestHandler extends Handler
{
@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
super.handleMessage(msg);
System.out.println("Handler running :"+msg.what+"!");
mTextView.setText(String.valueOf(msg.what));
}

}
}

當然，總的來說Java還是不推薦使用static變數的，這本身也不符合物件導向的變成思想，所以，建議除了一些final值，盡量還是多使用訊息機制來解決問題，維護也輕鬆些