"JDK11故障排除指南-发布版本11E94880-07"

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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.

Make a Java Application Easier to Debug

Using a logging framework is a good way to enable future debugging.

Chapter 1

Gather Relevant Data

1-3

Diagnostic Tools Overview

Most of the command-line utilities described in this section are either included in the

JDK or native operating system tools and utilities.

Although the JDK command-line utilities are included in the JDK download, it is

important to consider that they can be used to diagnose issues and monitor

applications that are deployed with the Java Runtime Environment (JRE).

In general, the diagnostic tools and options use various mechanisms to get the

information they report. The mechanisms are specific to the virtual machine (VM)

implementation, operating systems, and release. Frequently, only a subset of the tools

is applicable to a given issue at a particular time. Command-line options that are

prefixed with

-XX

are specific to Java HotSpot VM. See Java HotSpot VM Command-

Line Options.

Note:

The -XX options are not part of the Java API and can vary from one release

to the next.

The tools and options are divided into several categories, depending on the type of

problem that you are troubleshooting. Certain tools and options might fall into more

than one category.

• Postmortem diagnostics These tools and options can be used to diagnose a

problem after an application crashes. See Postmortem Diagnostic Tools.

• Hung processes These tools can be used to investigate a hung or deadlocked

process. See Hung Processes Tools.

• Monitoring These tools can be used to monitor a running application. See

Monitoring Tools.

• Other These tools and options can be used to help diagnose other issues. See

Other Tools, Options, Variables, and Properties.

Note:

Some command-line utilities described in this section are experimental. The

jstack

jinfo

, and

jmap

utilities are examples of utilities that are

experimental. It is suggested to use the latest diagnostic utility,

jcmd

instead

of the earlier

jstack

jinfo,

and

jmap

utilities.

JDK Mission Control

JDK Mission Control (JMC) is a production-time profiling and diagnostics tool. It

includes tools to monitor and manage your Java application with very small

performance overhead.

Chapter 2

Diagnostic Tools Overview

2-2

JMC's very small performance overhead is a result of its tight integration with the HotSpot

VM. JMC functionality is always available on-demand, and its small performance overhead is

only in effect while the tools are running. This approach also eliminates the problem of

the observer effect, which occurs when monitoring tools alter the execution characteristics of

the system. JMC enables you to troubleshoot issues and identify root causes and

bottlenecks. These properties make the JMC tool ideal for applications running in production.

JMC consists of the following client applications and plug-ins :

• JVM Browser shows running Java applications and their JVMs.

• JMX Console is a mechanism for monitoring and managing JVMs. It connects to a

running JVM, collects, displays its characteristics in real time, and enables you to change

some of its runtime properties through Managed Beans (MBeans). You can also create

rules that trigger on certain events (for example, send an e-mail if the CPU usage by the

application reaches 90 percent).

• Flight Recorder (JFR) is a tool for collecting diagnostic and profiling data about a running

Java application. It is integrated into the JVM and causes very small performance

overhead, so it can be used in production environments. JFR continuously saves large

amounts of data about the running applications. This profiling information includes thread

samples, lock profiles, and garbage collection details. JFR presents diagnostic

information in logically grouped tables and charts. It enables you to select the range of

time and level of detail necessary to focus on the problem. Data collected by JFR can be

essential when contacting Oracle support to help diagnose issues with your Java

application.

• jcmd Utility or Diagnostic Commands is used to send diagnostic command requests to

the JVM. These requests are useful for managing recordings from Flight Recorder,

troubleshooting, and diagnosing JVM and Java applications.

• Plug-ins help in heap dump analysis and DTrace recording. See Plug-in Details. JMC

plug-ins connect to a JVM using the Java Management Extensions (JMX) agent. For

more information about JMX, see the Java Platform, Standard Edition Java Management

Extensions Guide .

Troubleshoot with JDK Mission Control

JMC provides the following features or functionalities that can help you in troubleshooting:

• 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 from the JMC tool provide troubleshooting activities.

• Flight Recording in JMC 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 garbage collection, threads, and I/O. The Automated Analysis Results page of

flight recordings helps you to diagnose issues quicker. The provided rules and heuristics

help you find functional and performance problems in your application and provide tuning

tips. Some rules that operate with relatively unknown concepts, like safe points, will

provide explanations and links to further information. Some rules are parametrized and

can be configured to make more sense in your particular environment. Individual rules

can be enabled or disabled as you see fit.

– Flight Recorder in the JMC application 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.

Chapter 2

JDK Mission Control

2-3

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"JDK11故障排除指南-发布版本11E94880-07"

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