Can't create handler inside thread that has not called Looper.prepare()
2013-04-10 16:01
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不长记性,总是犯这个错误——在一个新建的线程中进行了Toast的操作
默认情况下Android中新诞生的线程是没有开启消息循环的。(主线程除外,主线程系统会自动为其创建Looper对象,开启消息循环)。座椅如果在主线程中开启一个handler,用message传递消息来在主线程handler中进行Toast是可行的,也是我一般的作法,现在来探寻下如果不用主线程的话应该怎么解决。
首先确定问题的产生原因是什么,
Toast或者Dialog中都有一个Handler的成员变量,在初始化时都会跟着初始化,而Toast或者Dialog中的Handler都需要一个Looper,所以需要在包含该Toast或者Dialog的线程中(如下面的Timer线程)初始化Looper。Looper.prepare();
Handler可看做是Looper的一个接口,用来向指定的Looper发送消息及定义处理方法。
以下转自 http://jeff-pluto-1874.iteye.com/blog/869710
寻找 Toast、Looper 和 Handler 三者之间的联系,也许谜底就能解开了。欲揭谜底,从源码入手是一条捷径。
Toast.java 的第231行的代码是创建一个新的Toast实例,而实例化的过程中,就需要执行第68行,也就是声明并创建Handler(成员变量)的实例。那么来看Handler.java的第121行到底做了什么,如下所示:
Java代码
119 mLooper = Looper.myLooper();
120 if (mLooper == null) {
121 throw new RuntimeException(
122 "Can't create handler inside thread that has not called Looper.prepare()");}
到此,距离真相的解开近了一大步,既然抛出了 RuntimeException,那么 mLooper 肯定是 null,但是为什么 Looper.myLooper() 会返回 null?继续进入到 Looper.java 中寻根究底。
Java代码
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static final Looper myLooper() {
return (Looper)sThreadLocal.get();
}
以上就是 myLooper() 方法的真实面貌,通过注释可以看出问题的真正原因在于当前线程并没有绑定 Looper,返回为 null 是正确但非正常的结果。
那么正确的调用方法应该是什么呢
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
总而言之,Toast-->初始化Handler-->Handler实例化的过程中调用mLooper = Looper.myLooper();-->当前线程并没有绑定
Looper,返回为 null -->Handler抛出异常Can't create handler inside thread that has not called Looper.prepare()
默认情况下Android中新诞生的线程是没有开启消息循环的。(主线程除外,主线程系统会自动为其创建Looper对象,开启消息循环)。座椅如果在主线程中开启一个handler,用message传递消息来在主线程handler中进行Toast是可行的,也是我一般的作法,现在来探寻下如果不用主线程的话应该怎么解决。
首先确定问题的产生原因是什么,
Toast或者Dialog中都有一个Handler的成员变量,在初始化时都会跟着初始化,而Toast或者Dialog中的Handler都需要一个Looper,所以需要在包含该Toast或者Dialog的线程中(如下面的Timer线程)初始化Looper。Looper.prepare();
Handler可看做是Looper的一个接口,用来向指定的Looper发送消息及定义处理方法。
以下转自 http://jeff-pluto-1874.iteye.com/blog/869710
寻找 Toast、Looper 和 Handler 三者之间的联系,也许谜底就能解开了。欲揭谜底,从源码入手是一条捷径。
Toast.java 的第231行的代码是创建一个新的Toast实例,而实例化的过程中,就需要执行第68行,也就是声明并创建Handler(成员变量)的实例。那么来看Handler.java的第121行到底做了什么,如下所示:
Java代码
119 mLooper = Looper.myLooper();
120 if (mLooper == null) {
121 throw new RuntimeException(
122 "Can't create handler inside thread that has not called Looper.prepare()");}
到此,距离真相的解开近了一大步,既然抛出了 RuntimeException,那么 mLooper 肯定是 null,但是为什么 Looper.myLooper() 会返回 null?继续进入到 Looper.java 中寻根究底。
Java代码
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static final Looper myLooper() {
return (Looper)sThreadLocal.get();
}
以上就是 myLooper() 方法的真实面貌,通过注释可以看出问题的真正原因在于当前线程并没有绑定 Looper,返回为 null 是正确但非正常的结果。
那么正确的调用方法应该是什么呢
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
总而言之,Toast-->初始化Handler-->Handler实例化的过程中调用mLooper = Looper.myLooper();-->当前线程并没有绑定
Looper,返回为 null -->Handler抛出异常Can't create handler inside thread that has not called Looper.prepare()
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