最近我在Android Studio 3.3中发现了同样的问题。我关闭了其他Android工作室项目，Logcat开始工作。上面这个公认的答案根本不符合逻辑。

使用Log类。输出与LogCat可见

Correction: On the emulator and most devices System.out.println gets redirected to LogCat and printed using Log.i(). This may not be true on very old or custom Android versions. Original: There is no console to send the messages to so the System.out.println messages get lost. In the same way this happens when you run a "traditional" Java application with javaw. Instead, you can use the Android Log class: Log.d("MyApp","I am here"); You can then view the log either in the Logcat view in Eclipse, or by running the following command: adb logcat It's good to get in to the habit of looking at logcat output as that is also where the Stack Traces of any uncaught Exceptions are displayed. The first Entry to every logging call is the log tag which identifies the source of the log message. This is helpful as you can filter the output of the log to show just your messages. To make sure that you're consistent with your log tag it's probably best to define it once as a static final String somewhere. Log.d(MyActivity.LOG_TAG,"Application started"); There are five one-letter methods in Log corresponding to the following levels: e() - Error w() - Warning i() - Information d() - Debug v() - Verbose wtf() - What a Terrible Failure The documentation says the following about the levels: Verbose should never be compiled into an application except during development. Debug logs are compiled in but stripped at runtime. Error, warning and info logs are always kept.

如果你真的需要System.out.println工作(例如。它是从第三方库调用的)。你可以简单地使用反射来更改System.class中的out字段:

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(); 
}

RedirectLogOutputStream类:

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;
        }
    }
}

System.out.println("…")会显示在Android Studio的Android Monitor上

最近我在Android Studio 3.3中发现了同样的问题。我关闭了其他Android工作室项目，Logcat开始工作。上面这个公认的答案根本不符合逻辑。