Print and view logs in Java - Function Compute - Alibaba Cloud - Function Compute

Function Compute collects all output written to stdout or stderr into the Simple Log Service (SLS) Logstore you specified when creating the service. This topic explains how to print structured logs from a Java function, read the execution summary that Function Compute appends to every invocation, and view logs in the console.

Print logs

Two options are available for printing logs from a Java function:

Method	When to use
`context.getLogger` — provided by the fc-java-core library	Use this when you need to correlate logs with a specific invocation. Each log entry includes a timestamp, request ID, and log level, so you can filter the Logstore for all output from a single function call.
Third-party logging libraries (for example, logback)	Use these when your application already has an established logging setup.

We recommend that you use the context.getLogger method. Logs printed by this method contain request IDs, which you can use to query logs and troubleshoot errors.

Use context.getLogger

The following example writes an INFO-level log entry and returns a response:

package example;

import com.aliyun.fc.runtime.Context;
import com.aliyun.fc.runtime.StreamRequestHandler;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class HelloFC implements StreamRequestHandler {

    @Override
    public void handleRequest(InputStream inputStream, OutputStream outputStream, Context context) throws IOException {
        context.getLogger().info("hello world");
        outputStream.write(new String("hello world").getBytes());
    }
}

Running this handler produces the following output in your Logstore:

message:2017-07-05T05:13:35.920Z a72df088-f738-cee3-e0fe-323ad891**** [INFO] hello world

Each log entry contains the time, request ID, and log level followed by your message.

Log levels

context.getLogger supports six log levels. Choose the level that matches the intent of the message:

Log level	Method	Standard usage
TRACE	`context.getLogger().trace`	Fine-grained execution path details; most verbose
DEBUG	`context.getLogger().debug`	Detailed information for diagnosing system behavior
INFO	`context.getLogger().info`	Normal operation messages
WARN	`context.getLogger().warn`	Potential issues that may cause unexpected behavior if unaddressed
ERROR	`context.getLogger().error`	Failures that prevent the code from running as expected
FATAL	`context.getLogger().fatal`	Critical errors that stop the application; least verbose

Understand the execution summary

For every invocation, Function Compute records FC Invoke Start and FC Invoke End marker lines and appends an execution summary to the log. The summary contains the following fields:

Field	Description
Request ID	The request ID of the invocation.
Code Check Value	The verification code of the function code package used for this invocation.
Function Execution Time	The time the function handler actually took to run.
Function Billing Time	The amount of time you are charged for.
Function Memory	The memory allocated to the function.
Actual Used Memory	The maximum memory used during the invocation.

View logs

After a function runs, view its logs on the Logs tab of the Function Details page in the Function Compute console. For step-by-step instructions, see View function invocation logs.