4장. Class Loader

Java Performance
artdb@ex-em.com | performeister.tistory.com | twitter @novathinker

Class Loader

Java Performance
2


ClassLoader
1) Dynamic Linking
2) Namespace
3) Delegation Model
4) Classloader Tree in WAS
ClassLoader Work
1) ClassLoader Work
2) Loading
3) Linking
(1) Veriﬁcation
(2) Preparation
(3) Resolution
4) Initialization

Java Performance
4
Java Performance Fundamental | twitter @novathinker
artdb@ex-em.com | performeister.tistory.com
ClassLoader

• Dynamic Linking
– Runtime Class
– Class Load
Link
– Link Classloader
– JVM
• Bootstrap Classloader
• Java Runtime Library (rt.jar)
• Object class

Java Performance
5
ClassLoader

• Dynamic Linking
– Loadtime dynamic loading
• class

public class Hello {
public static void main(String[] args) {
System.out.println(“Hello EXEM”);
}
}
• Java.lang.String, java.lang.System Hello class
loading loading

Java Performance
6
ClassLoader

• Dynamic Linking
– Runtime dynamic loading (reﬂection)
• Code

public class Hello {
public static void main(String[] args) {
Class cl = Class.forName(args[0]);
}
}
• Class.forName(args[0]) args[0]
Class

Java Performance
7
ClassLoader

• Namespace
– JVM Class Loading

– Class ?

1. Full Qualiﬁed Name .
Full Qualiﬁed Name = package name + class name
2. Classloader .

Java Performance
8
ClassLoader

• Namespace
– Class Loader Namespace

Classloader Classloader
1 2
Namespace
Namespace
exem.package.jvmclass

exem.package.jvmclass exem.package.jvmclass

Java Performance
8
ClassLoader

• Namespace
– Class Loader Namespace

1 2
Namespace
Namespace
exem.package.jvmclass exem.package.jvmclass


Java Performance
9
ClassLoader

• Namespace
– Symbolic Reference
– Class Load classloader
class loading

Class exemHello {
Class cl = Class.forName(exem.package.jvmclass);
}

1 2
Namespace

exemHello


Java Performance
10
Classloader

• Delegation Model
– Classloder

Java Performance
11
Classloader

– Classloader
• Bootstrap classloader
: classloader (no Parent)
: native implementation
: $JAVA_HOME/jre/lib/rt.jar
• Extension classloader
: class
: $JAVA_HOME/jre/lib/ext jar ﬁle
• System classloader
: Application classloader
: $CLASSPATH, java.class.path

Java Performance
12
Classloader

– Delegation Model
• classloader bootstrap
parent
• Class CacheParentSelf
• Class loading (delegation)
• classloader default system
classloader
• Parent system
classloader child
•

Java Performance
13
Classloader

– Class Loading
• Classloader loadClass()
(1) Class
(2) Load parent
(3) Parent findClass() call
• Classloader findClass()
– Current Classloader Class
• Delegation Class
– bootstrapextensionsystemuser-defined
– Class bootstap
extension class path

Java Performance
14
Classloader

– Class Loading
• Classloader loadClass()
(1) Class
(2) Load parent
(3) Parent findClass() call
• Classloader findClass()
– Current Classloader Class
• Delegation Class
– bootstrapextensionsystemuser-defined
– Class bootstap
extension class path

Java Performance
15
Classloader

• Classloader Tree in WAS
– EJB, WAR deploy classloader

• Isolating the Application from other
Applications
• Sharing Class between deployed modules
• Implementing multiple classloaders

Java Performance
16
Classloader


Java Performance
17
Classloader


Java Performance
18
Classloader

• Class Sharing
– Jvm Load Class
– ClassLoader CacheParentShared Class File
System

Shared Class

Class
JVM1

JVM2

JVM3

Sharing
JVM1

JVM2

JVM3

Java Performance
19
Classloader

• Class Sharing
– Hotspot JVM
• Shared Archive
• Jar Class JVM Memory
Mapped File
• Shared Library
• Client VM, Default Garbage Collector

Java Performance
20
Classloader

• Class Sharing
– IBM JVM
• Class Cache

• Class Cache JVM – JVM
Cache
• JVM Cache
• Cache
• Cache OS

Java Performance
21
Classloader

– Path Parameter
• -Xbootclasspath:<path(s) or file(s)>
: Bootstrap classloader
• -Xbootclasspath/a:<path(s) or file(s)>
: boot classpath
• -Xbootclasspath/p:<path(s) or file(s)>
: boot classpath
• -Djava.ext.dirs=<path(s) or file(s)>
: Extension classloader
• -cp, -classpath <path(s) or file(s)>
-Djava.class.path=<path(s) or file(s)>
: System classloader

Java Performance

Class Loader Work

Java Performance
23
Classloader Work

• Classloader Work

Link

Load Verify

Prepare

Resolve Initialize

Java Performance
24
Classloader Work

• Classloader Work
– JVM class ﬁle loading, linking,
Initializing Runtime Memory Type

• Loading : binary type JVM

• Linking : JVM Runtime Binary type
data

– Veriﬁcation : JVM
– Preparation: Type Memory
– Resolution : constant pool symbolic reference

Java Performance
25
Classloader Work

• Loading
– Loading Process
• Acquistion
: java class ﬁle format ﬁle, network

• Parse
: method area parse
• java.lang.class instance
: class instance application jvm internal
data interface

Java Performance
26
Classloader Work

• Linking
– Verification
• Java language (semantic)
• Verification Vendor
• JVM Verification
• Exception
• Parse verification

– File size
– Superclass
• Verification Linking

Java Performance
27
Classloader Work

• Linking
– Veriﬁcation
•
– Final class subclass

– Final class override

– Abstract method

– Type superclass method ,

Java Performance
28
Classloader Work

• Linking
– Veriﬁcation
• Superclass, superinterface load
– constant pool

– Constant pool Class, Method
,Method descriptor String

– Bytecode (integrity)

»

»

Java Performance
29
Classloader Work

• Linking
– Preparation
• , ,

• Class default

• Method table data

– Method table : Superclass method superclass

Java Performance
30
Classloader Work

• Linking
– Preparation
Variable Type Default Value
int 0
long 0L
short (short)0
char ‘u0000’
byte (byte)0
reference null
ﬂoat 0.0f
double 0.0d
Boolean

Java Performance
30
Classloader Work

• Linking
– Preparation
Variable Type Default Value
int 0
long 0L
short (short)0
char ‘u0000’
byte (byte)0
reference null
ﬂoat 0.0f
double 0.0d
Boolean Boolean primitive type
int (false default)

Java Performance
31
Classloader Work

• Linking
– Resolution
• Type constant pool class, interface,
method, ﬁeld symbolic reference direct
reference

• Eager lazy
– Loading Linking JVM
– , vendor

Java Performance
32
Classloader Work

• Linking
– Resolution
• Eager lazy
– Eager : JVM Application Class
Loading, Linking
– Lazy : First Actual Use Loading, Linking
– Vendor Lazy
– 1.4 Eager
» Hotspot : -XX:+eagerinitializtion (1.4.1)
» IBM : -Dibm.cl.eagerresolution (1.4.2)

Java Performance
33
Classloader Work

• Linking
– Resolution
• Actual Use
– Explicit loading
– Actual Use of class
» Class new instance
» Element type class array
» Class method
» Final static class

Java Performance
34
Classloader Work

• Linking
– Resolution
• Actual Use
– Actual Use of interface
» Interface

• compile
Passive Use implicit
loading

Java Performance
35
Classloader Work

• Initialization
– initial value class
• Value

– Class Initialization Step
• superclass initialized
superclass initializing
• Class class initialization method <clinit()

Java Performance
36
Classloader Work

• Initialization
– Interface Initialization Step
• Interface superinterface initialization
initialization method

– Loading, Linking Initialization time

• First Active Use Initialization

• Loading, Linking Initialization

Java Performance
37
Classloader Work

• Initialization
– Synchronization

• Thread initialization
Thread Thread
Wait

• Initialization Thread notify

Java Performance
38

4장. Class Loader

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4장. Class Loader