Shulou(Shulou.com)06/03 Report--

This article will explain in detail how to deeply analyze the dump file in jvm. The content of the article is of high quality, so the editor will share it for you as a reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

JVM dump

Java memory dump is a snapshot of jvm runtime memory, which can be used to analyze whether there is memory waste, to check whether memory management is reasonable, and to find out the cause of the problem when OOM occurs. So what is the content of the dump file? Let's take it one step at a time.

Get the JVM dump file

The ways to obtain dump files are active and passive.

i. Active approach:

1. Using jmap is also the most commonly used way: jmap-dump: [live], format=b,file=

two。 Using jcmd,jcmd GC.heap_dump

3. Using VisualVM, you can operate the dump memory through the interface.

4. Through the JMX way

MBeanServer server = ManagementFactory.getPlatformMBeanServer (); HotSpotDiagnosticMXBean mxBean = ManagementFactory.newPlatformMXBeanProxy (server, "com.sun.management:type=HotSpotDiagnostic", HotSpotDiagnosticMXBean.class); mxBean.dumpHeap (filePath, live)

ii. Passive mode:

Passive mode is our usual OOM event, by setting the parameter-XX:+HeapDumpOnOutOfMemoryError-XX:HeapDumpPath=

Dump file analysis

Structural diagram

Detailed explanation of structure

The dump file is a mapping of heap memory, which consists of a header and a series of content blocks

File header

It is composed of four parts: musk, version, identifierSize and time.

1. Musk:4 byte with the contents of 'Jacks,' Aids, 'Vics,' which means JAVA

2. Version: several byte, including the following three values

"PROFILE 1.0\ 0", "PROFILE 1.0.1\ 0", "PROFILE 1.0.2\ 0"

3. IdentifierSize:4 byte digits with a value of 4 or 8, indicating the number of byte occupied by a reference

4. Time to generate time:8 byte,dump files

Description: there are two types of member variables in a java class.

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

Basic types (8 basic types), which occupy a fixed number of byte. Each time an object is generated, they need to assign initial values and allocate space to them.

Is a reference type that represents an object, there is only one reference in the class, the reference is just a numeric value, the space occupied is identifierSize, and the referenced object is about to be in another place in the heap

For example, define a class

Public class Person {private int age;//4 byte private String name;//identifierSize byte private double weight;//8 byte}

When we were in new Person ()

It needs to apply for a space, which is the object header size + 4+identifierSize+8 byte.

Measurement of object size:

Jdk provides a tool Instrumentation for the amount of memory occupied by test objects, but Instrumentation cannot be referenced directly, so it needs to be referenced through agent.

Define a Premain class, javac Premain.java

/ / Premain.java public class Premain {public static java.lang.instrument.Instrumentation inst; public static void premain (String args, java.lang.instrument.Instrumentation inst) {Premain.inst = inst;}}

Write a Manifest file

Manifest.mf Manifest-Version: 1.0 Premain-Class: Premain Can-Redefine-Classes: true Can-Retransform-Classes: true

Packing

Jar-cmf manifest.mf premain.jar Premain.class

Define an execution class, javac PersonTest.java

/ / PersonTest.java public class PersonTest {public static void main (String [] args) throws Exception {Class clazz = Class.forName ("Premain"); if (clazz! = null) {Person p = new Person (); java.lang.instrument.Instrumentation inst = (java.lang.instrument.Instrumentation) clazz.getDeclaredField ("inst") .get (null) System.out.println ("person size: [" + inst.getObjectSize (p) + "] B"); System.out.println ("class size: [" + inst.getObjectSize (p.getClass ()) + "] B");}

Execute with agent

Java-javaagent:premain.jar PersonTest

Results:

Person size: [32] B class size: [504] B

Content block

Each block is made up of blocks and blocks.

Block

The block header consists of a block type of 1 byte, and a time of 4 byte the length of time,4 byte indicates the number of byte occupied by this content block

There are generally five types of type, string, class, stack frame, stack, and dump block

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

String, consisting of the string id of identifierSize byte, followed by the string contents of (length-identifierSize) byte (the id in which the string is directly referenced later)

Class, consisting of 4 byte class sequences (used in the stack frame), identifierSize byte class id (used when parsing classes), 4 byte sequence id (not used yet), identifierSize byte class name id

Stack frame, identifierSize byte id,identifierSize byte method name id,identifierSize byte method identification id,identifierSize byte class file name id,4 byte class sequence, 4 byte line numbers

Stack, consisting of 4 byte stack sequence numbers, 4 byte thread sequence numbers, 4 byte frames, followed by several identifierSize byte frame id

Dump block is the content of all objects. Each object consists of 1 subtype of byte, which is combined with object content. There are 6 subtypes, gc root, thread object, class, object, basic type array, object array.

Gc root

There are 4 structures and 8 types of gc root.

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

IdentifierSize object id of byte, with SYSTEM_CLASS,BUSY_MONITOR and non-UNKNOWN types

IdentifierSize byte object id,4 byte thread sequence number, type NATIVE_STACK,THREAD_BLOCK

IdentifierSize byte object id,4 byte thread sequence number, 4 byte stack frame depth, type JAVA_LOCAL,NATIVE_LOCAL

IdentifierSize byte object id,identifierSize byte global refId (not in use), with type NATIVE_STATIC

Gc root schematic diagram

Gc root is the source of garbage collection traceability. Each gc root points to an initial object, and untraceable objects are to be recycled.

System class, only the class whose classLoader is null is gc root, and each class is a gc root

Thread stack, thread method parameters, local variables are all gc root, and each object is a gc root

The system retains objects, each of which is a gc root

Class object

1. Basic information:

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

IdentifierSize class object id of byte

Stack sequence number of 4 byte

Id, the parent object of identifierSize byte

IdentifierSize classLoader object id of byte

IdentifierSize Signer object id of byte

IdentifierSize protection domain object id of byte

Reserved id1 and id2 for identifierSize byte

Class instance object size of 4 byte

The number of constants of 2 byte, followed by each constant, 2 subscript of byte, 1 constant type of byte, and the content of several byte, depending on the type (boolean/byte is 1 byte, char/short is 2 byte,float/int, 4 byte, double/long is 8 byte, reference type is identifierSize byte)

10. The number of static variables of 2 byte, followed by the variable name id of each static variable, the variable name of identifierSize byte, the variable type of 1 byte, and the content of several byte, which is determined according to the type (see Article 9 of basic information on class objects)

11. The number of member variables of 2 byte, followed by each member variable, identifierSize byte variable name id,1 byte variable type

2. Description:

(1) the constants in the class are not used in many places, so the number of constants is generally 0.

(2) the name, type and value of the static variable of the class are placed in the class object, and the name and type of the member variable are also placed in the class object, but the value of the instance is placed in the instance object.

Instance object

1. Basic information:

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

IdentifierSize instance object id of byte

Stack sequence number of 4 byte

IdentifierSize class id of byte

Number of bytes occupied by 4 byte

The value of the variable of the instance

2. Description:

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

The value of the instance is the member variable value of the instance object, the order is the variable value of the current class, the order is the order in Article 11 of the basic information of the class object, and then the variable value of the parent class.

The basic type of the value of a variable has a default value, and the default value of the reference type is 0, which occupies the number of bytes (see Article 9 of the basic information of class objects)

Basic type array

1. Basic information:

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

Array object id of identifierSize byte

Stack sequence number of 4 byte

Array length of 4 byte

Element type of 1 byte

List of values for the element

2. Description:

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

The value of the element (see article 9 for basic information of class objects)

Object array

1. Basic information:

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

Array object id of identifierSize byte

Stack sequence number of 4 byte

Array length of 4 byte

Element class id of identifierSize byte

List of values for the element

Memory allocation

When a thread starts, the process goes to system memory to generate a thread stack

Whenever a method call occurs, a stack frame is pressed into the stack, and when the method call is finished, the stack frame exits.

In the process of running, if there is an object's new operation, the process will go to the heap area to request a piece of memory

For more information about runtime memory, you can find relevant information

Memory recovery rule

If an object cannot ride gc root reference reachability, then the object may have to be recycled

Object collection rules include

Hongmeng official Strategic Cooperation to build HarmonyOS Technology Community

Instance properties are referenced by the instance, and can be recycled only when the instance is recycled (for strong references only).

Class objects are referenced by instances, and only when all instances of a class are recycled can the class be recycled.

The parent class of the class object, the classLoader object, the signer object, and the protection domain object are referenced by the class. Only when the class is recycled can these be recycled.

The scope of a local variable (in the thread stack) is a curly braces

Public void test () {Object a = new Object (); / / obj 1 Object b = new Object (); / / obj 2 {Object c = new Object (); / / obj 3 a = null;//obj 1 can be recycled} / / obj 3 can be recycled} / / obj 2 can be recycled

Brief introduction of analysis tools

To analyze the dump file, we can use the jhat tool provided in jdk to execute

Jhat xxx.dump

Jhat loads and parses xxx.dump files, and opens a simple web service. The default port is 7000. You can view some statistical information in memory through the browser.

General usage

1. Open http:/127.0.0.1:7000 in browser

Some features are listed, including an overview of the various classes under package, and each feature navigation

2. Heap memory statistics of clicked pages

There is a table, the type of object, the number of instances, the amount of memory occupied by the instance, and which type of object takes up the most memory at a glance.

3. Click the name of the class that consumes too much memory to view the details of the class.

It mainly shows the size of each instance below the class, as well as some link navigation

4. Click references summary by type

If there are too many objects of a certain type, it is possible that there are too many objects in the class that references it.

Basically, some simple page queries, combined with the original code, can initially locate the memory leak.

On how to in-depth analysis of the dump file in jvm to share here, I hope that the above content can be of some help to you, can learn more knowledge. If you think the article is good, you can share it for more people to see.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.