blocks|key|1335375|text|使用Log+class。输出通过LogCat可见|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1335376|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.android.com/reference/android/util/Log.html^0|2|9|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Use the <a href="http://developer.android.com/reference/android/util/Log.html" rel="noreferrer">Log class</a>. Output visible with LogCat

blocks|key|1120262|text|是的，确实如此。如果您正在使用仿真器，它将显示在System.out标记下的Logcat视图中。编写一些代码并在您的模拟器中进行尝试。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1120263|entityMap^0|O|A|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

Yes it does. If you're using the emulator, it will show in the Logcat view under the <code>System.out</code> tag. Write something and try it in your emulator.

blocks|key|1335468|text|当然，要在logcat中查看结果，您应该至少将Log+level设置为"Info“(Log+level+in+logcat)；否则，就像我遇到的那样，您将看不到输出。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1335469|entityMap|0|LINK|mutability|MUTABLE|url|http://i.stack.imgur.com/hWCH7.png^0|16|J|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Of course, to see the result in logcat, you should set the Log level at least to "Info" (<a href="http://i.stack.imgur.com/hWCH7.png" rel="nofollow">Log level in logcat</a>); otherwise, as it happened to me, you won't see your output.

blocks|key|1120481|text|如果你真的需要System.out.println来工作(例如，它是从第三方库调用的)。您可以简单地使用反射来更改System.class中的字段：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1120482|try{
++++Field+outField+=+System.class.getDeclaredField("out");
++++Field+modifiersField+=+Field.class.getDeclaredField("accessFlags");
++++modifiersField.setAccessible(true);
++++modifiersField.set(outField,+outField.getModifiers()+&+~Modifier.FINAL);
++++outField.setAccessible(true);
++++outField.set(null,+new+PrintStream(new+RedirectLogOutputStream());+
}catch(NoSuchFieldException+e){
++++e.printStackTrace();+
}catch(IllegalAccessException+e){
++++e.printStackTrace();+
}|code-block|syntax|javascript|1120483|RedirectLogOutputStream类：|1120484|public+class+RedirectLogOutputStream+extends+OutputStream{
++++private+String+mCache;

++++@Override
++++public+void+write(int+b)+throws+IOException{
++++++++if(mCache+==+null)+mCache+=+"";

++++++++if(((char)+b)+==+'\n'){
++++++++++++Log.i("redirect+from+system.out",+mCache);
++++++++++++mCache+=+"";
++++++++}else{
++++++++++++mCache+%2B=+(char)+b;
++++++++}
++++}
}|1120485|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

if you really need System.out.println to work(eg. it's called from third party library). you can simply use reflection to change out field in System.class:

<pre><code>try{
 Field outField = System.class.getDeclaredField("out");
 Field modifiersField = Field.class.getDeclaredField("accessFlags");
 modifiersField.setAccessible(true);
 modifiersField.set(outField, outField.getModifiers() &amp; ~Modifier.FINAL);
 outField.setAccessible(true);
 outField.set(null, new PrintStream(new RedirectLogOutputStream()); 
}catch(NoSuchFieldException e){
 e.printStackTrace(); 
}catch(IllegalAccessException e){
 e.printStackTrace(); 
}
</code></pre>

RedirectLogOutputStream class:

<pre><code>public class RedirectLogOutputStream extends OutputStream{
 private String mCache;

 @Override
 public void write(int b) throws IOException{
 if(mCache == null) mCache = "";

 if(((char) b) == '\n'){
 Log.i("redirect from system.out", mCache);
 mCache = "";
 }else{
 mCache += (char) b;
 }
 }
}
</code></pre>

blocks|key|1335817|text|它不会显示在您的应用程序中...它在模拟器的logcat下|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1335818|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

it is not displayed in your application... it is under your emulator's logcat

blocks|key|1335575|text|你的手机上没有任何地方可以阅读System.out.println();|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1335576|相反，如果你想看到某件事的结果，可以查看你的logcat/console窗口，或者在设备屏幕上显示Toast或Snackbar+(如果你使用的是较新的设备)，并显示消息:)例如，当我必须检查它在switch+case代码中的位置时，我就会这样做！祝您编码愉快！:)|1335577|entityMap^0|F|L|0|M|E|1D|5|1J|8|2P|B|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|I|$]]

There is no place on your phone that you can read the <code>System.out.println();</code>

Instead, if you want to see the result of something either look at your <code>logcat/console</code> window or make a <code>Toast</code> or a <code>Snackbar</code> (if you're on a newer device) appear on the device's screen with the message :) 
That's what i do when i have to check for example where it goes in a <code>switch case</code> code! Have fun coding! :)

blocks|key|1120512|text|System.out.println("...")显示在Android+Studio的Android监视器上|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1120513|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

System.out.println("...") is displayed on the Android Monitor in Android Studio

blocks|key|1120573|text|我会把这个留给更多的访问者，因为对我来说，这是关于主线程无法System.out.println的问题。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1120574|public+class+LogUtil+{

private+static+String+log+=+"";
private+static+boolean+started+=+false;
public+static+void+print(String+s)+{
++++//Start+the+thread+unless+it's+already+running
++++if(!started)+{
++++++++start();
++++}
++++//Append+a+String+to+the+log
++++log+%2B=+s;
}

public+static+void+println(String+s)+{
++++//Start+the+thread+unless+it's+already+running
++++if(!started)+{
++++++++start();
++++}
++++//Append+a+String+to+the+log+with+a+newline.
++++//NOTE:+Change+to+print(s+%2B+"\n")+if+you+don't+want+it+to+trim+the+last+newline.
++++log+%2B=+(s.endsWith("\n")+)?+s+:+(s+%2B+"\n");
}

private+static+void+start()+{
++++//Creates+a+new+Thread+responsible+for+showing+the+logs.
++++Thread+thread+=+new+Thread(new+Runnable()+{
++++++++@Override
++++++++public+void+run()+{
++++++++++++while(true)+{
++++++++++++++++//Execute+100+times+per+second+to+save+CPU+cycles.
++++++++++++++++try+{
++++++++++++++++++++Thread.sleep(10);
++++++++++++++++}+catch+(InterruptedException+e)+{
++++++++++++++++++++e.printStackTrace();
++++++++++++++++}
++++++++++++++++//If+the+log+variable+has+any+contents...
++++++++++++++++if(!log.isEmpty())+{
++++++++++++++++++++//...print+it+and+clear+the+log+variable+for+new+data.
++++++++++++++++++++System.out.print(log);
++++++++++++++++++++log+=+"";
++++++++++++++++}
++++++++++++}
++++++++}
++++});
++++thread.start();
++++started+=+true;
}
}|code-block|syntax|javascript|1120575|用法：LogUtil.println("This+is+a+string");|1120576|entityMap^0|U|I|0|0|3|10|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

I'll leave this for further visitors as for me it was something about the main thread being unable to <code>System.out.println</code>.

<pre><code>public class LogUtil {

private static String log = "";
private static boolean started = false;
public static void print(String s) {
 //Start the thread unless it's already running
 if(!started) {
 start();
 }
 //Append a String to the log
 log += s;
}

public static void println(String s) {
 //Start the thread unless it's already running
 if(!started) {
 start();
 }
 //Append a String to the log with a newline.
 //NOTE: Change to print(s + "\n") if you don't want it to trim the last newline.
 log += (s.endsWith("\n") )? s : (s + "\n");
}

private static void start() {
 //Creates a new Thread responsible for showing the logs.
 Thread thread = new Thread(new Runnable() {
 @Override
 public void run() {
 while(true) {
 //Execute 100 times per second to save CPU cycles.
 try {
 Thread.sleep(10);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 //If the log variable has any contents...
 if(!log.isEmpty()) {
 //...print it and clear the log variable for new data.
 System.out.print(log);
 log = "";
 }
 }
 }
 });
 thread.start();
 started = true;
}
}
</code></pre>

Usage: <code>LogUtil.println("This is a string");</code>

blocks|key|1120637|text|最近，我在Android+Studio+3.3中注意到了同样的问题。我关闭了其他Android+studio项目，Logcat开始工作。上面被接受的答案根本不符合逻辑。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1120638|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Recently I noticed the same issue in Android Studio 3.3. I closed the other Android studio projects and Logcat started working. The accepted answer above is not logical at all.

I want to print something in console, so that I can debug it. But for some reason, nothing prints in my Android application. 

How do I debug then?

<pre><code>public class HelloWebview extends Activity {
 WebView webview; 
 private static final String LOG_TAG = "WebViewDemo";
 private class HelloWebViewClient extends WebViewClient {
 @Override
 public boolean shouldOverrideUrlLoading(WebView view, String url) {
 view.loadUrl(url);
 return true;
 }
 }

 /** Called when the activity is first created. */
 @Override
 public void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.main);
 webview = (WebView) findViewById(R.id.webview);
 webview.setWebViewClient(new HelloWebViewClient());
 webview.getSettings().setJavaScriptEnabled(true);
 webview.setWebChromeClient(new MyWebChromeClient());
 webview.loadUrl("http://example.com/"); 
 System.out.println("I am here");
 }
</code></pre>

Why doesn't "System.out.println" work in Android?

我想在控制台中打印一些东西，这样我就可以调试它了。但由于某种原因，我的Android应用程序中什么都没有打印出来。那我该怎么调试呢？public class HelloWebview extends Activity {    WebView webview;        private static final String LOG_TAG = "WebViewDemo";    privat

问为什么"System.out.println“在安卓上不起作用？
EN

回答 9

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么"System.out.println“在安卓上不起作用？EN

回答 9

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么"System.out.println“在安卓上不起作用？
EN