JDK17故障排除指南 - 功能、发布信息及版权

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sometimes disabled by default. To enable core files on Linux, it is usually enough

to run the

ulimit -c unlimited

before starting the application command. Some

systems may have different ways to handle these limits.

Note:

The core files take up a lot of disk space, especially when enabled.

To decide whether to enable core files, consider what you would do if you had a

crash in your system. Would you want to see a core file? Many Java users won't

have much use for a core file. However, if you would want to debug a possible

crash either in a native debugger such as

gdb

or by using the Serviceability Agent,

then ensure that you enable core files before the starting the application.

Many times, crashes are hard to reproduce; therefore, enable core files before the

starting the application.

2. Add -XX:+HeapDumpOnOutOfMemoryError to the JVM flags: The

XX:+HeapDumpOnOutOfMemoryError

flag saves a Java heap dump to disk if the

applications runs into an

OutOfMemoryError

Like core files, heap dumps can be very large, especially when run with a big Java

heap.

Again, think about what you would do if the application runs into an

OutOfMemoryError

. Would you want to inspect the heap at the time of the error? In

that case, enable this option so that you get this data if the application runs into an

unexpected

OutOfMemoryError

. (The option is disabled by default.)

3. Run a continuous flight recording: Set up Java to run with a continuous flight

recording.

Continuous flight recordings are a circular buffer of Flight Recorder events. If the

application runs into an issue, you can dump the data from the last hour of the run.

The Flight Recorder events can be helpful to debug a wide range of issues from

memory leaks to network errors, high CPU usage, thread blocks, and so on.

The overhead of running with a continuous flight recording is very low. See

Produce a Flight Recording.

4. Add -verbosegc to the JVM command-line: The flag

-verbosegc

logs basic

information about Java Garbage Collector. This log helps you find the following:

• Does garbage collection run for a long time?

• Does the free memory decrease over time?

The garbage collector log helps diagnose issues when the application throws an

OutOFMemoryError

or the application runs into performance issues; therefore,

turning on the

-verbosegc

flag helps troubleshoot issues.

Chapter 1

Enable Options and Flags for JVM Troubleshooting

1-2

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 running it. Another alternative is to add

XX+PrintCommandLineFlags

and

-showversion

to the JVM arguments.

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

remotely using tools such as JDK 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

– 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 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

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 a very small

performance overhead, and is suitable for monitoring applications running in

production.

JMC is not part of the regular JDK installation. For more information on JMC

downloads and documentation, see JDK Mission Control Page.

JMC consists of :

• 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

Chapter 2

JDK Mission Control

2-2

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