Bite-sized ByteCode and ClassLoaders

@alblue
©2020 Alex Blewitt
Bite Sized Bytecode
Classes and ClassLoaders

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@alblue
©2020 Alex Blewitt
Overview
• ClassLoaders

• Classes

• Bytecode

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@alblue
©2020 Alex Blewitt
Class Loaders and Class files

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@alblue
©2020 Alex Blewitt
Loading and defining classes
• The JVM builds Class instances from .class ﬁles

• A class loader is responsible for ﬁnding (or generating) bytes

• Class.forName("an.example") → triggers lookup if not loaded

• A ClassLoader can be chained to a 'parent classloader'

• Most app servers (e.g. Tomcat, Netty) have one ClassLoader per app
App 1
CL
App 2
CL
App 3
CL
App 4
CL
Tomcat CL
ClassLoaders also load resources!

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@alblue
©2020 Alex Blewitt
ClassLoaders and Classes
• A Class is owned by its loading ClassLoader

• A Class must be uniquely named in a ClassLoader

• Two Class objects with the same name can exist in a JVM

• A name is not unique; a Class+ClassLoader pair is unique

a.getClass().getName().equals("ClassName") == true;
(ClassName)a ! ClassCastException
• Could be caused by multiple web apps storing thread local variables

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@alblue
©2020 Alex Blewitt
Loading a class
• The mechanisms to bring a class into existence vary

• URLClassLoader can load classes from a URL

• AppletClassLoader (was) used to load applets into a browser

• ASM, ByteBuddy, Mockito etc. generating classes on the ﬂy

• In essence, custom class loaders boil down to:

1. Load or generate bytes from somewhere

2. Call defineClass()

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@alblue
©2020 Alex Blewitt
Dynamic class creation
• Classes can be created at runtime, from Java 1.3 onwards:

final Runnable r = (Runnable)
Proxy.newProxyInstance(getClass().getClassLoader(),
new Class>[] { Runnable.classv},
(InvocationHandler) (instance, method, args) -> {
System.out.println("Hello World!");
return null;
}
);
• Easier to use a lambda for Java 8 and above:

Runnable r = () -> { System.out.println("Hello World!"); };

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@alblue
©2020 Alex Blewitt
Tool
Provider
System
JavaC
File
Manager
Source
File
Class File
Class
Loader

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@alblue
©2020 Alex Blewitt
• Can also compile and load classes programmatically: 
var javac = javax.tools.ToolProvider.getSystemJavaCompiler();
var fileMgr = javac.getStandardFileManager(null, null, null);
var srcs = fileMgr.getJavaFileObjects("/tmp/Test.java");
javac.getTask(null, fileMgr, null, null, null, srcs).call();
var cl = new ClassLoader() {
public Class load(final byte[] bytes) {
return defineClass(bytes, 0, bytes.length);
}
};
final var bytes = Files.readAllBytes(Path.of("/tmp/Test.class"));
((Runnable) (cl.load(bytes).newInstance())).run();
public class Test implements Runnable {
public void run() {
System.out.println("Hello World!");
}
}
https://github.com/alblue/jvmulator/blob/master/src/main/java/com/bandlem/jvm/jvmulator/compiler/

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@alblue
©2020 Alex Blewitt
Class file format
Magic
0xcafebabe
Minor
0
Major
55
Constant Pool
count
Flags
public
This Super
Fields
count
Methods
count
Class Attributes
count
UTF8
Count
5
E x a m p e
l
Int
0x48656c6f
Float
0x7f800000
Long
0x416c2042_6c756521
Doubl
0xfff00000_00000000
Class
UTF8
1
Field
Class
2
NaT
3
Method
Class
2
NaT
4
NaT
Name
1
Type
6
String
UTF8
1
IMethod
Class
2
NaT
4
UTF8
Count
2
[ Z
1⃣ 1⃣
8⃣ 7⃣
3⃣
4⃣ 6⃣
5⃣
Name
1
Type
6
Flags
public
Attributes
count
Attribute
Data
Length
Name
1
Attributes allow
for extensions
The ﬁle format hasn't
changed for decades
Few constant types
have been added

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@alblue
©2020 Alex Blewitt
Attributes
• Class ﬁles can be adorned with many attributes

• Extensible, stringly-typed array of bytes

• Code → contains bytecode for a method's execution

• Exceptions → list of exceptions that can be thrown form a method

• Runtime(In)visibleAnnotations → set of key/value pairs

• NestHost/NestMembers → new support for nest mates in Java 11 (JEP 181)

• Attributes are optional; e.g. native/abstract methods have no "Code" attribute

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@alblue
©2020 Alex Blewitt
Special methods
• Classes can have 'special' or synthetic methods

• – method that runs when the class is accessed at ﬁrst time

• – constructor special name

• Accessor methods generated for inner classes

• Have ACC_SYNTHETIC set, so they don't show up in tools

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@alblue
©2020 Alex Blewitt
Displaying bytecode
• Java has a built-in disassembler for Java code

javap -c [-p[rivate]] [-v[erbose]] [-cp classpath] com.Example[.class]
Disassemble
byte code
Show all private
members (c.f.
proteted, package,
public
Display
constant pool
and attributes
Classpath (or —
module-path) of
class name
Class name
(optional .class
extension)

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@alblue
©2020 Alex Blewitt
JavaP – displaying bytecode
$ javap -v -c java.lang.Object
public class java.lang.Object
minor version: 0
major version: 55
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #17 // java/lang/Object
super_class: #0
interfaces: 0, fields: 0, methods: 14, attributes: 1
Constant pool:
#1 = Class #63 // java/lang/StringBuilder
#2 = Methodref #1.#64 // java/lang/StringBuilder."":()V
#3 = Methodref #17.#65 // java/lang/Object.getClass:()Ljava/lang/Class;
#4 = Methodref #66.#67 // java/lang/Class.getName:()Ljava/lang/String;
...
#6 = String #69 // @
...
#17 = Class #80 // java/lang/Object 
... 
#34 = Utf8 equals
#35 = Utf8 (Ljava/lang/Object;)Z
...
#80 = Utf8 java/lang/Object
All other classes will have a
super_class which is not 0
Compiled against Java 11
Constant used in default 'toString' method
Used to deﬁne equals(Object) method

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@alblue
©2020 Alex Blewitt
JavaP – displaying bytecode
$ javap -v -c java.lang.Object
public class java.lang.Object
public boolean equals(java.lang.Object);
descriptor: (Ljava/lang/Object;)Z
flags: (0x0001) ACC_PUBLIC
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: aload_1
2: if_acmpne 9
5: iconst_1
6: goto 10
9: iconst_0
10: ireturn
LineNumberTable:
line 158: 0
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this Ljava/lang/Object;
0 11 1 obj Ljava/lang/Object;
SourceFile: Object.java
Code attribute
LineNumberTable (nested) attribute
LocalVariableTable (nested) attribute
Used to deﬁne equals(Object) method
Slot 0 usually contains 'this'
Slot 1 contains the ﬁrst argument obj
Line number 158
of Object.java
Source attribute

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@alblue
©2020 Alex Blewitt
Bytecode for equals()
Code:
0: aload_0
1: aload_1
2: if_acmpne 9
5: iconst_1
6: goto 10
9: iconst_0
10: ireturn
Locals Stack
0
1
➡
this
other

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@alblue
©2020 Alex Blewitt
Code:
0: aload_0
1: aload_1
2: if_acmpne 9
5: iconst_1
6: goto 10
9: iconst_0
10: ireturn
Locals Stack
0 this this
1 other
➡

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@alblue
©2020 Alex Blewitt
Code:
0: aload_0
1: aload_1
2: if_acmpne 9
5: iconst_1
6: goto 10
9: iconst_0
10: ireturn
Locals Stack
0 this this
1 other other
➡

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@alblue
©2020 Alex Blewitt
Code:
0: aload_0
1: aload_1
2: if_acmpne 9
5: iconst_1
6: goto 10
9: iconst_0
10: ireturn
Locals Stack
0 this
1 other
➡

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@alblue
©2020 Alex Blewitt
Code:
0: aload_0
1: aload_1
2: if_acmpne 9
5: iconst_1
6: goto 10
9: iconst_0
10: ireturn
Locals Stack
0 this
1 other
➡
0

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@alblue
©2020 Alex Blewitt
Code:
0: aload_0
1: aload_1
2: if_acmpne 9
5: iconst_1
6: goto 10
9: iconst_0
10: ireturn
➡
0

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@alblue
©2020 Alex Blewitt
Bytecode

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@alblue
©2020 Alex Blewitt
Bytecode
• Most bytecodes are encoded as a single byte (hence the name)

• Some bytecodes take additional operands, but most operate on the stack

• Bytecodes can:

• Consume values from the stack

• Push a value onto the stack

• Transfer from the stack to a local variable (and vice versa)

• Load constants from the class' constant pool

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@alblue
©2020 Alex Blewitt
Reference and object bytecodes
• new – push a new instance of the class from the constant pool

• newarray – push a new array with a primitive type Z B C S I L

• anewarray – push a new array of reference types

• multianewarray – push a multi-dimensional array

• arraylength – push the length of the array

• checkcast – throw if top of stack is not of the speciﬁed type

• instanceof – push true if top of stack is of speciﬁed type
Array of booleans is [Z
Array of array of char is [[C

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@alblue
©2020 Alex Blewitt
Calling methods
• invokestatic – call a static method (constant contains class)

• invokevirtual – call instance methods of ToS (with inheritance)

• invokespecial – call super constructor/methods of ToS

• invokeinterface – call an interface method on ToS

• invokedynamic – invokes a dynamic method (since Java 1.7)

→ Used for implementing Lambda operations

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@alblue
©2020 Alex Blewitt
Mathematics
• {i,l,f,d}neg – negates the top of stack

• {i,l,f,d}add/sub – adds/subtracts two numbers together

• {i,l,f,d}mul/div – multiplies/divides one number from the other

• {i,l,f,d}rem – remainder when divided by (modulus)

• {i,l}and/or/xor – performs bitwise and/or/xor on two numbers

• {i,l}shl/shr/ushr – arithmetic shift left/right or unsigned (bitwise) shift right
Consumes top two
stack items, pushes
result onto stack
Consumes and pushes
single element on stack

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@alblue
©2020 Alex Blewitt
Constants
• {i,l,f,d}const_{0,1} – push 0 or 1 onto the stack as integer/long/ﬂoat/double

• iconst_{2,3,4,5,m1} – push 2,3,4,5 or -1 onto the stack as an integer

• {b,s}ipush – push the next byte/short onto the stack

• ldc{,_w,2_w} – push a constant from the pool onto the stack

• aconst_null – push 'null' on to the stack

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@alblue
©2020 Alex Blewitt
Conversions
int
short
char
byte
long
ﬂoat double
d2f
f2d
f2l d2i
l2f
f2i
i2f l2d
d2l
i2d
i2l l2i
i2b
i2s
i2c
6⃣4⃣
3⃣2⃣
8⃣
1⃣6⃣
1⃣6⃣
3⃣2⃣ 6⃣4⃣
boolean

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@alblue
©2020 Alex Blewitt
Loading and storing
• {b,s,c,i,f,l,d,a}aload/astore – load/store element into array at index

• {i,l,f,d,a}load/store{,_0,_1,_2,_3} – load/store from variable at index

• iinc – increment local variable by constant byte

• getfield/putfield <field> – get/put a field in an instance on ToS

• getstatic/putstatic <field> – get/put a static field in a class

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@alblue
©2020 Alex Blewitt
Comparisons
• {f,d}cmpg – compare two floats/doubles, pushes 1 on NaN

• {f,d}cmpl – compares two floads/dobules, pushes -1 on NaN

• lcmp – compares two longs, pushes 1 or -1

• if{eq,ne,gt,ge,lt,le} <±jump> – branch if =, ≠, >, ≥, <, ≤ 0

• if_icmp{eq,ne,gt,ge,lt,le} <±jump> – branch if =, ≠, >, ≥, <, ≤ other number

• if_acmp{eq,ne} <±jump> – branch if references are equal or not equal

• if{,non}null <±jump> – branch if (non) null
IEEE754 floating point spec
uses 'Not a Number' to
represent conditions such as
divide-by-zero or sqrt(-1)

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@alblue
©2020 Alex Blewitt
Control flow
• {lookup,table}switch – continue execution from table (switch)

• {,i,l,f,d,a}return – return a void/int/long/ﬂoat/double/reference

• goto{_w} <±jump> – jump to another bytecode (do not push address)

• athrow – throw the (Throwable) reference on top of the stack

• jsr{_w} <±jump> – jump to another part of the method (push address)

• ret – return (from a jsr) to an address speciﬁed in local var

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@alblue
©2020 Alex Blewitt
Stack manipulation
• swap – swap the top two int/float values on the stack

• pop{,2} – pop (drop) one or two slots from the stack

• dup – duplicate the top int/float on the stack

• dup_x{1,2} – duplicate the top int/float on the stack, put it 1 or 2 below

• dup2 – duplicate the top long/double on the stack

• dup2_x{1,2} – duplicate the top long/double on the stack, put it 1 or 2 below

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@alblue
©2020 Alex Blewitt
Miscellaneous
• nop – no operation

• monitor{enter,exit} – synchronized blocks

• breakpoint – breakpoint for debuggers

• impdep{1,2} – implementation dependent operations for debuggers

• wide – treat the next bytecode as having wider argument

• iinc → wide iinc

• *load/*store/ret → wide *load/*store/ret

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@alblue
©2020 Alex Blewitt
Summary
• Java class files define a class, along with methods and fields

• ClassLoader instances loads a class from somewhere (disk, url, …) as a Class

• Methods' implementation are bytecodes stored in Code attributes

• Bytecode operates on a stack, with a number of 'local' variables

• The stack and locals operate on int/long/float/double/reference types

• Conversions between data types are handled with opcodes

• Some opcodes take operands but the majority do not

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@alblue
©2020 Alex Blewitt
Thank you
https://alblue.bandlem.com

https://twitter.com/alblue

https://github.com/alblue

https://vimeo.com/alblue

https://speakerdeck.com/alblue

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Bite-sized ByteCode and ClassLoaders

Bite-sized ByteCode and ClassLoaders

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