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This article mainly shows you "what are the knowledge points of Java bytecode", which is easy to understand and clear. I hope it can help you solve your doubts. Let me lead you to study and learn this article "what are the knowledge points of Java bytecode?"

Compile the "1x 1" code

First of all, we need to write a simple Mini Program, 1x1 program, learning from the simplest 1x1, the code is as follows:

Package top.luozhou.test;/** * @ description: * @ author: luozhou * @ create: 2019-12-25 21:28 * * / public class TestJava {public static void main (String [] args) {int astat1; System.out.println (a);}}

After writing the java class file, first execute the command javac TestJava.java to compile the class file and generate TestJava.class. Then execute the decompilation command javap-verbose TestJava, and the bytecode result is shown as follows:

Compiled from "TestJava.java" public class top.luozhou.test.TestJava minor version: 0 major version: 56 flags: ACC_PUBLIC, ACC_SUPERConstant pool: # 1 = Methodref # 5.314 / / java/lang/Object. "": () V # 2 = Fieldref # 15.016 / / java/lang/System.out:Ljava/io/PrintStream # 3 = Methodref # 17 / 18 / java/io/PrintStream.println: (I) V # 4 = Class # 19 / / top/luozhou/test/TestJava # 5 = Class # 20 / / java/lang/Object # 6 = Utf8 # 7 = Utf8 () V # 8 = Utf8 Code # 9 = Utf8 LineNumberTable # 10 = Utf8 main # 11 = Utf8 ([Ljava/lang/String ) V # 12 = Utf8 SourceFile # 13 = Utf8 TestJava.java # 14 = NameAndType # 6 Class / / "": () V # 15 = Class # 21 / / java/lang/System # 16 = NameAndType # 22 / / out:Ljava/io/PrintStream # 17 = Class # 24 / / java/io/PrintStream # 18 = NameAndType # 25 println 26 / / println: (I) V # 19 = Utf8 top/luozhou/test/TestJava # 20 = Utf8 java/lang/Object # 21 = Utf8 java/lang/System # 22 = Utf8 out # 23 = Utf8 Ljava/io/PrintStream # 24 = Utf8 java/io/PrintStream # 25 = Utf8 println # 26 = Utf8 (I) V {public top.luozhou.test.TestJava () Descriptor: () V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial # 1 / / Method java/lang/Object. "": () V 4: return LineNumberTable: line 8: 0 public static void main (java.lang.String []); descriptor: ([Ljava/lang/String ) V flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=2, args_size=1 0: iconst_2 1: istore_1 2: getstatic # 2 / / Field java/lang/System.out:Ljava/io/PrintStream 5: iload_1 6: invokevirtual # 3 / / Method java/io/PrintStream.println: (I) V 9: return LineNumberTable: line 10: 0 line 11: 2 line 12: 9} parsing bytecode

1. Basic information

The above results remove some redundant information that does not affect the parsing, and then we will parse the results of the bytecode.

Minor version: 0 version number, 0 indicates that major version: 56 major version number is not used, 56 indicates jdk12, indicating that flags: ACC_PUBLIC, ACC_SUPER can only be run in the jdk12 version and later virtual machines

ACC_PUBLIC: this is an access flag of type public or not.

ACC_SUPER: this falg is designed to solve the problem of calling the super method through the invokespecial instruction. It can be understood as a bug patch for Java 1.0.2, and only in this way can it correctly find super class methods. Starting with Java 1.0.2, the compiler always generates an ACC_SUPER access identity in bytecode. Interested students can click here to learn more.

two。 Constant pool

Next, we will analyze the constant pool, which you can also understand against the overall bytecode above.

# 1 = Methodref # 5.14 / / java/lang/Object. "": () V

This is a method reference, where # 5 represents the index value, and then we can find that the bytecode with an index value of 5 is as follows

# 5 = Class # 20 / / java/lang/Object

It indicates that this is an Object class, just as # 14 points to a "": () V that refers to an initialization method.

# 2 = Fieldref # 15.16 / / java/lang/System.out:Ljava/io/PrintStream

The above paragraph represents a field reference, which also references # 15 and # 16, actually referring to the PrintStream object in the java/lang/System class. Other constant pool analysis ideas are the same, in view of the length, I will not explain them one by one, just list a few of the key types and information.

NameAndType: this representation is a constant table of names and types, and can point to the method name or the index of the field, which is the actual method of representation in the bytecode above.

Utf8: we often use character encoding, but this doesn't just mean character encoding, it represents a string whose character encoding is Utf8. It is the most commonly used table structure in virtual machines, and you can understand that it can describe methods, fields, classes and other information. For example:

# 4 = Class # 19 # 19 = Utf8 top/luozhou/test/TestJava

This means that there is a class under the index of # 4, and then the class that points to is # 19, which is a Utf8 table, and finally stores top/luozhou/test/TestJava, so you can know that the class referenced in position # 4 is top/luozhou/test/TestJava.

3. Construction method information

Next, we analyze the bytecode of the constructor. We know that when a class initializes, it first executes its constructor. If you do not write a constructor, the system will add a constructor without arguments to you by default.

Public top.luozhou.test.TestJava (); descriptor: () V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial # 1 / / Method java/lang/Object. "": () V 4: return LineNumberTable: line 8: 0

Descriptor: () V: indicates that this is a method with no return value.

Flags: ACC_PUBLIC: is a public method.

Stack=1, locals=1, args_size=1: indicates that the number in the stack is 1, the variable in the local variable table is 1, and the call parameter is also 1.

Why are there all ones here? Isn't this the default construction method? Where did you get the parameters? In fact, there is a hidden rule in Java language: in any instance method, the object to which this method belongs can be accessed through this. The implementation of this mechanism is through the Java compiler in the compilation time as an input parameter into the method, students familiar with the python language will know that defining a method in python always passes a parameter of self, which is also passed into the method internal reference of this instance, Java just pushes this mechanism back to the compilation phase to complete. So, the 1 here refers to the parameter this.

0: aload_0 1: invokespecial # 1 / / Method java/lang/Object. "": () V 4: return LineNumberTable: line 8: 0

After the above analysis, the meaning of this construction method is very clear.

Aload_0: means to load the first variable in the local variable table into the stack, that is, this.

Invokespecial: call the initialization method directly.

Return: the method ends when the call is completed.

LineNumberTable: this is a table of lines that records the mapping between the bytecode offset and the number of lines of code. Line 8: 0 indicates that line 8 of the source code corresponds to the bytecode with an offset of 0, which is not intuitively reflected here because it is the default construction method.

In addition, the constructor of Object is executed here because Object is the parent class of all classes, and the construction of subclasses requires the constructor of the parent class first.

4.main method information

Public static void main (java.lang.String []); descriptor: ([Ljava/lang/String;) V flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=2, args_size=1 0: iconst_2 1: istore_1 2: getstatic # 2 / / Field java/lang/System.out:Ljava/io/PrintStream 5: iload_1 6: invokevirtual # 3 / / Method java/io/PrintStream.println: (I) V 9: return LineNumberTable: line 10: 0 line 11: 2 line 12: 9

With the analysis of the previous construction method, we will be familiar with the next analysis of the main method. I will skip the repetition and focus on the code part.

Stack=2, locals=2, args_size=1: here the stack and local variables are 2, and the parameters are still 1. Why is that? Because a variable an is declared in the main method, the local variables are added to the table, and so is the stack, so they are 2. Then why is args_size still 1? Didn't you say that this will be passed in by default? It's supposed to be 2. Note: what I said earlier is that in any instance method, this main method is a static method, which can be accessed directly through the class + method name, and does not require an instance object, so there is no need to pass it in here.

0: iconst_2: push int type 2 to the top of the stack.

1: istore_1: stores the int type value at the top of the stack in the second local variable.

2: getstatic # 2 / / Field java/lang/System.out:Ljava/io/PrintStream;: gets the PrintStream class.

5: iload_1: push the second int local variable to the top of the stack.

6: invokevirtual # 3 / / Method java/io/PrintStream.println: (I) V: call the println method.

9: return: call the end method.

The LineNumberTable here has source code, so we can compare whether my previous description is correct:

Line 10: 0: 10 of line indicates 0: iconst_2 bytecode, and here we find that the compiler has directly calculated for us and pushed 2 to the top of the stack.

Line 11: 2: 11 of the line source code corresponds to the static class PrintStream that 2: getstatic gets the output.

Line 12: 9:12 line source code corresponds to return, indicating the end of the method.

Here I also draw a dynamic picture to demonstrate the execution of the main method, hoping to help you understand:

These are all the contents of the article "what are the knowledge points of Java bytecodes?" Thank you for your reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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