"JDK12故障排除指南：Java平台标准版故障排除指南"

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Note:

Use log rotation so that an application restart doesn't delete the previous

logs. Since JDK7, the flags

UseGClogFileRotation

and

NumberOfGCLogFiles

can be used to set up for log rotation. For a

description of these flags, see Debugging Options for Java HotSpot VM.

5. Print Java version and JVM flags: Before filing a bug on Java or seeking help

from a forum, have the basic information handy in the log files. For example, it's

helpful to print the Java version and the JVM flags used.

If your application starts with a script, run

java -version

to print the Java version

and print the command line before executing it. Another alternative is to add

-XX

+PrintCommandLineFlags

and

-showversion

to the JVM arguments.

6. Set up JMC JMX for remote monitoring: JMX can be used to connect to a Java

application remotely using tools such as Mission Control or Visual VM. Unless you

can run these tools on the same machine that is running your application, setting

this up can be helpful later on to monitor the application, send diagnostic

commands, manage flight recordings, and so on. There is no performance

overhead if you enable JMX.

Another alternative, is to enable JMX after a Java application has started is to use

the diagnostic command

ManagementAgent.start

. Run

jcmd <pid> help

ManagementAgent.start

for a list of flags that can be sent with the command.

See The jcmd Utility.

Gather Relevant Data

If your application runs into a problem and you want to debug the problem further,

ensure that you collect any relevant data before restarting the system, especially if

restarting will remove previous files.

• It is important to gather the following files:

– Core files for crash issues.

–

hs_err

printed text file for Java crashes.

– Log files: Java and application logs.

– Java heap dumps for

-XX:+HeapDumpOnOutOfMemoryError

– Java flight recordings (if enabled). If the problem didn't terminate the

application, dump the continuous recordings.

• If the application stopped responding, then gather the following files:

– Stack traces: Take several stack traces using

jcmd <pid> Thread.print

before restarting the system.

– Dump flight recordings (if enabled).

– Force a core file: If the application can't be closed properly, then stop the

application, and force a core file using

kill -6 <pid>

on Linux or Solaris

systems.

Chapter 1

Gather Relevant Data

1-3

Troubleshoot with Java Mission Control

Troubleshooting activities that you can perform with Java Mission Control.

Java Mission Control allows you to perform the following troubleshooting activities:

• Java Management console (JMX) connects to a running JVM, and collects and

displays key characteristics in real time.

• Triggers user-provided custom actions and rules for JVM.

• Experimental plug-ins like -

WLS

DTrace

JOverflow

, and others from the JMC tool

provide troubleshooting activities.

–

DTrace

plug-in is an extended

DScript

language to produce self-describing

events. It provides visualization similar to Java Flight Recorder.

–

JOverflow

is another plug-in tool for analyzing heap waste (empty/sparse

collections). It is recommended to use JDK 8 release and later for optimal use

of the

JOverflow

plug-in.

• The Java Flight Recording (JFR) in Java Mission Control is available to analyze

events. The preconfigured tabs enable you to easily to drill down in various areas

of common interest, such as, code, memory and gc, threads, and I/O. The General

Events tab and Operative Events tab together allow drilling down further and

rapidly honing in on a set of events with certain properties. The Events tab usually

has check boxes to only show events in the Operative set.

– JFR, when used as a plug-in for the JMC client, presents diagnostic

information in logically grouped tables, charts, and dials. It enables you to

select the range of time and level of detail necessary to focus on the problem.

• The Java Mission Control plug-ins connect to JVM using the Java Management

Extensions (JMX) agent. The JMX is a standard API for the management and

monitoring of resources such as applications, devices, services, and the Java

Virtual Machine.

What Are Java Flight Recordings

The Java Flight Recorder records detailed information about the Java runtime and the

Java application running in the Java runtime.

The recording process is done with little overhead. The data is recorded as time-

stamped data points called events. Typical events can be threads waiting for locks,

garbage collections, periodic CPU usage data, etc.

When creating a flight recording, you select which events should be saved. This is

called a recording template. Some templates only save very basic events and have

virtually no impact on performance. Other templates may come with slight

performance overhead, and may also trigger GCs in order to gather additional

information. In general, it is rare to see more than a few percentage of overhead.

Flight Recordings can be used to debug a wide range of issues from performance

problems to memory leaks or heavy lock contention.

The following topic describes types of recording to produce a Java flight recording.

• Types of Recordings

Chapter 2

What Are Java Flight Recordings

2-3

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"JDK12故障排除指南：Java平台标准版故障排除指南"

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