Questions about the JVM .class bytecode format or instruction set.
Questions tagged [jvm-bytecode]
188 questions
6
votes
2 answers
Is java byte-code always forward compatible?
I understand that the byte code generated by version JDK X is guaranteed to work on JVM Y, provided Y >= X.
Does this hold good for all versions of JDK/JVM?
i.e Is it fair to expect class files generated by JDK 1 to work on JVM 11?
Referred to JVM…
Sarvesh
- 519
- 1
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6
votes
1 answer
Java 8: Merge String[] and int[] into Object[] when verifying bytecode
I'm reading section 4.10.2.2 of the specification for JVM version 8 (https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html) how to verify bytecode. In this case what happens in control flow, when the stack now contains a slot with an int[]…
Emil
- 16,784
- 2
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6
votes
3 answers
Writing languages for the JVM
Suppose I write a programming language; for namesake, I'll call it lang.
To begin the long journey of writing lang, I decide to begin, by writing lang in itself. I can't actually run it, because theres nothing to run the program that runs itself.
So…
user5549921
5
votes
1 answer
What's the value of the generated getClass invocation for method references?
I have the following contrived code example. It does nothing useful, in order to keep the bytecode small, but hopefully you can see how, with some changes, it might.
List letters = Arrays.asList("a", "b");
Stream.of(/*a, b, c,…
Michael
- 41,989
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- 128
5
votes
1 answer
Switch expression with help of arrow (->), and now can yield/return the value
The extended switch expression in Java 14, the need of switch expression is unclear other than visual clarity for programmer/reviewer. Is it
a different byte code implementation than older switch expression?
any performance improvement in terms of…
Jatish
- 352
- 1
- 2
- 10
5
votes
1 answer
Why does AspectJ generate an empty Annotation check?
I'm using AspectJ 1.8.8 compile-time weaving and I have a block like this
@SomeAnnotation(value="someValue")
public List doSomething(String someArg) {
...
}
where @SomeAnnotation is implemented with an "Around" advice.
Looking at the bytecode…
Lycus
- 410
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5
votes
2 answers
How does bipush work in JVM?
I understand iload takes in integers -1 to 5, but how can you extend to higher numbers using a bipush instruction? How is the specific integer being stored with the bytecode?
Tim
- 138
- 3
- 8
5
votes
1 answer
Java Bytecode Signatures
As part of the compiler for the programming language I am working on, I came across generic signatures in the bytecode, which I am trying to parse and convert to an AST. The parsing algorithm mostly works, but there seems to be a special case in…
Clashsoft
- 11,553
- 5
- 40
- 79
5
votes
2 answers
"throws" and "annotation for exception" in Dalvik bytecode
Why the byte code is generated as
.annotation system Ldalvik/annotation/Throws;
value = {
Ljava/io/FileNotFoundException;
}
.end annotation
rather than .throws Ljava/io/FileNotFoundException
if a method declares…
monica
- 1,035
- 1
- 9
- 21
4
votes
1 answer
Why does COMPUTE_FRAMES generate a lot of redundant instructions?
I'm writing a compiler that outputs JVM byte code, and using ASM 9.4 for the backend. This works fine, but I am puzzled about one particular quirk.
I'm specifying COMPUTE_FRAMES to automatically compute the stack frames. It is said that the tradeoff…
rwallace
- 31,405
- 40
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4
votes
2 answers
Does lambda () => 1 result in object creation at run time each time it is passed as an argument?
My understanding was that all non-capturing lambdas shouldn't require object creation at use site, because one can be created as a static field and reused. In principle, the same could be true for lambdas constituting of a class method call - only…
Turin
- 2,208
- 15
- 23
4
votes
2 answers
java JVM bytecode notation, comment grammar. InvokeDynamic
Question: What does line 14 means?
Use javap -v -c to disassembly the following code:
public class test {
static int i = 2;
public static void main(String[] args) {
test x = new test();
System.out.println("text +…
schen
- 107
- 1
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- 7
4
votes
1 answer
Performance benefits of recompiling old java project with the latest java version
Does it make sense to recompile a project with a new (java 11) target version from a performance perspective if it will be run on a java 11 while source code will remain the same (java 8)?
org.apache.maven.plugins
…
Serge
- 2,574
- 18
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4
votes
2 answers
Constructor bytecode
The ASM guide talks about constructors:
package pkg;
public class Bean {
private int f;
public int getF() {
return this.f;
}
public void setF(int f) {
this.f = f;
}
}
The Bean class also has a default public constructor which…
rapt
- 11,810
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4
votes
1 answer
Bytecode for Java string literals longer than 65535 bytes
I have been reading Java bytecode from a variety of files to help with my understanding of the .class files for a project where I need to integrate with a 3rd party library which has no source code and poor documentation available.
For my own…
Mr Rho
- 578
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