深入java虚拟机深入java虚拟机

Step 1. Magic Number hex bytes name --------- ---- CAFEBABE magic First the Java Virtual Machine (JVM) must make sure that the class file starts with the proper magic number. In this case the JVM will be happy because it will find the CafeBabe magic right where it's supposed to be. Note that, because all multi-byte values are stored in the class file in big-endian order, the first byte of every class file is 0xCA. **** Step 2. Version Numbers hex dec name --- --- ---- 0003 3 minor_version 002D 45 major_version Next, the JVM must make sure that it recognizes and fully understands the format of the class file being loaded. If either the major or minor version number is not among the version numbers for which this JVM was implemented, the JVM must reject the class file. In this case, the JVM will be relieved to find that the file has major version 45 and minor version 3, of which it has intimate knowledge. **** Step 3. Constant Pool Count 0011 17 constant_pool_count The next two bytes make up an unsigned short integer that indicates the number of entries in the constant pool. In this case the constant pool will have 17 entries, but because the zeroeth entry doesn't appear in the class file, the JVM will expect to find entries 1 through 16 next in the stream. ** Step 4. constant_pool[1] 07 7 tag 0007 7 name_index Each constant pool entry contains a table whose type is indicated by the first byte of the entry, the tag. The first constant pool entry is a CONSTANT_Class_info table. The JVM knows this because it finds a 7, which means CONSTANT_Class, in the tag byte position. Aside from the tag, the CONSTANT_Class_info table has only a name_index, which in this case is 7. This is an index into the constant pool. The JVM doesn't know it yet, because it hasn't read in the 7th entry of the constant pool, but the 7th entry will be the string "java/lang/Object", the name of the superclass of this class. Because class Act doesn't explicitly descend from any other class, it by default descends from class Object. Note that this is the first constant pool entry and that it already has index 1. constant_pool[0] doesn't appear in the class file. *** Step 5. constant_pool[2] 07 7 tag 0010 16 name_index The 2nd constant pool entry is another CONSTANT_Class_info table. The JVM knows this because it again finds a 7, which still means CONSTANT_Class, in the tag byte position. The name_index here, however, is 16. constant_pool[16] will be the string "Act", although once again the JVM doesn't know this yet because it hasn't read that far into the constant pool. At this point the JVM only knows that when it does get around to the 16th constant pool entry, that entry will contain the name of the class that is represented by the CONSTANT_Class_info table occupying constant_pool[2]. constant_pool[2] therefore represents the "this" class, class Act, which this class file defines. *** Step 6. constant_pool[3] 0A 10 tag 0002 1 class_index 0004 4 name_and_type_index constant_pool[3] contains a CONSTANT_Methodref_info table, indicated by the tag of 10, which means CONSTANT_Methodref. A CONSTANT_Methodref_info table represents a method and records its name, type, and the class to which it belongs. The first two bytes after the tag, the class_index, form an index into the constant pool, in this case a 2. constant_pool[2] represents class Act, so this method is declared in Act. The next two bytes make up the name_and_type_index, which is an index into the constant pool pointing to a CONSTANT_NameAndType table. In this case name_and_type_index is 4 and constant_pool[4] defines "void <init>()". Because class Act doesn't declare its own constructor, the javac compiler generates a default constructor, Act(). Act() becomes the instance initialization method <init>() in the class file. Act's <init>() method just calls <init>() in superclass Object. ***** Step 7. constant_pool[4] 0C 12 tag 0006 6 name_index 0005 5 descriptor_index constant_pool[4] is a CONSTANT_NameAndType_info table identified by its initial tag of 12, which stands for CONSTANT_NameAndType. Following the tag byte is the name_index, which is 6. constant_pool[6] will be the string "<init>". Next is the descriptor_index, which is 5. constant_pool[5] will be the string "()V". "<init>" is the name of the method being described. "()V" is the type. In plain Java it would look like "void <init>()". "()V" is a method descriptor. The "()" indicates that there are no arguments to "<init>". The "V" indicates the return type of "<init>" is void. ***** Step 8. constant_pool[5] 01 1 tag 0003 3 length 282956 "()V" bytes[length] constant_pool[5] is a CONSTANT_Utf8_info table that contains the string "()V", a method descriptor that translates to a method that takes no arguments and returns void. CONSTANT_Utf8 is how strings are encoded in the constant pool. The UTF-8 format allows any 16 bit character to be represented by either 1, 2, or 3 bytes. The characters '\u0001' to '\u007F' occupy only one byte. Other characters require 2 or 3 bytes, however, these characters are expected to appear less frequently than the characters that require only 1 byte. By this means the full 16 bit Unicode character set can be supported by the class file without using as much space as would likely be used by just making each character 16 bits long. The tag byte of a CONSTANT_Utf8_info table is followed by a length, in this case 13, which indicates the length in bytes of the UTF-8 format string that follows. In this case the string spells out "ConstantValue". This string is used internally by the class file in a manner that will be described later in the file loading simulation. There is no trailing null character in a CONSTANT_Utf8_info bytes item, as that would unnecessarily waste bandwidth. ****** Step 9. constant_pool[6] 01 1 tag 0006 6 length 3C696E69743E "<init>" bytes[length] constant_pool[6] is a CONSTANT_Utf8_info table that contains the string "<init>", the name of a method in the superclass, Object. ********* Step 10. constant_pool[7] 01 1 tag 0003 3 length 416374 "Act" bytes[length] constant_pool[7] is a CONSTANT_Utf8_info table that contains the string "Act". This is the name of the class that is being defined by this file. ****** Step 11. constant_pool[8] 01 1 tag 000B 11 length 736E697065742E6A617661 "Act.java" bytes[length] constant_pool[8] is a CONSTANT_Utf8_info table that contains the string "Act.java", the name of the source file in which class Act was defined. This string is referred to by a SourceFile attribute. *********** Step 12. constant_pool[9] 01 1 tag 0004 4 length 436F6465 "Code" bytes[length] constant_pool[9] is a CONSTANT_Utf8_info table that contains the string "Code", an attribute name. ******* Step 13. constant_pool[10] 01 1 tag 000D 13 length 436F6E7374616E7456616C7565 "ConstantValue" bytes[length] constant_pool[10] is a CONSTANT_Utf8_info table that contains the string "ConstantValue", an attribute name. **************** Step 14. constant_pool[11] 01 1 tag 000A 10 length 457863657074696F6E73 "Exceptions" bytes[length] constant_pool[11] is a CONSTANT_Utf8_info table that contains the string "Exceptions", an attribute name. ************* Step 15. constant_pool[12] 01 1 tag 000F 15 length 4C696E654E756D6265725461626C65 "LineNumberTable" bytes[length] constant_pool[12] is a CONSTANT_Utf8_info table that contains the string "LineNumberTable", an attribute name. ****************** Step 16. constant_pool[13] 01 1 tag 000E 14 length 4C6F63616C5661726961626C6573 "LocalVariables" bytes[length] constant_pool[13] is a CONSTANT_Utf8_info tabl