Using HiLog to Print Logs (ArkTS)

During application development, you can log from your application's key code. You can find out how your application is running through logs. For example, the logs can tell you whether the application is running correctly, whether the code running time sequence is normal, and whether the running logic and branch is normal).

To print logs, you can call HiLog or console APIs. This document covers HiLog APIs. For details about the console APIs, see console.

Available APIs

HiLog defines five log levels: DEBUG, INFO, WARN, ERROR, and FATAL, and provides the following APIs to output logs at different levels. For details, see hilog.

Parameters

NOTE

• The domain and tag specified in isLoggable() must be the same as those used in the logging API.

• The level specified in isLoggable() must match the logging API.

• domain: service domain of logs. The value range is 0x0000 to 0xFFFF. You can define the value as required.

• tag: log identifier. It can be any string. You are advised to use this parameter to identify the class or service behavior of a method call.

• level: log level. For details about the value, see LogLevel.

• format: format in which logs are output. The value is a string in the "%{private flag}specifier" format.

You can set several parameters in the format string. For example, %s in the format string %s World identifies a variable of the string type, whose value is defined in args.

To enable parameters identified by {private} to be displayed as they are, run hilog -p off.

• args: the parameter list corresponding to the parameter type specified in the format string. There can be no or multiple parameters in the list. The number and type of parameters must be mapped to the identifiers in the format string.

Constraints

A log contains a maximum of 4096 bytes. If this limit is exceeded, excess content will be discarded.

Example

This example shows how to log each click on the added button.

1. Create a project, with Empty Ability as the template.

2. On the project configuration page, set Model to Stage.

3. In the Project window, choose entry > src > main > ets > pages, open the Index.ets file, add a button, and enable logging upon a click on the button. The sample code is as follows:

   // Index.ets
   
   import hilog from '@ohos.hilog';
   
   @Entry
   @Component
   struct Index {
     build() {
       Row() {
         Column() {
           // Add a button to respond to user clicks.
           Button() {
             Text('Next')
               .fontSize(30)
               .fontWeight(FontWeight.Bold)
           }
           .type(ButtonType.Capsule)
           .margin({
             top: 20
           })
           .backgroundColor('#0D9FFB')
           .width('40%')
           .height('5%')
           // Bind the onClick event to the button and print logs when the button is clicked.
           .onClick(() => {
             hilog.isLoggable(0xFF00, "testTag", hilog.LogLevel.INFO);    
             hilog.info(0xFF00, "testTag", "%{public}s World %{public}d", "hello", 3);
           })
         }
         .width('100%')
       }
       .height('100%')
     }
   }
   

   The following is sample code for outputting an info-level log:

   "%{public}s World %{public}d"
   

   where %{public}s indicates a string whose privacy flag is public, and %{public}d an integer whose privacy flag is public.

4. Run the project on a real device, and touch Next on the app/service screen on the device.

5. At the bottom of DevEco Studio, switch to the Log tab and set the filter criteria. Specifically, select the current device and process, set the log level to Verbose, and enter testTag in the search box. Only the logs that meet the filter criteria are displayed in the window.

   In this example, the printed log is "hello World 3."