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The Code Attribute

The Code attribute is a variable-length attribute in the attributes table of a method_info structure (). A Code attribute contains the Java Virtual Machine instructions and auxiliary information for a method, including an instance initialization method or a class or interface initialization method ().

If the method is either native or abstract, its method_info structure must not have a Code attribute in its attributes table. Otherwise, its method_info structure must have exactly one Code attribute in its attributes table.

The Code attribute has the following format:

Code_attribute {    u2 attribute_name_index;    u4 attribute_length;    u2 max_stack;    u2 max_locals;    u4 code_length;  #  指令长度是多少    u1 code[code_length];   # 取code_length长度  指令    u2 exception_table_length;    {   u2 start_pc;        u2 end_pc;        u2 handler_pc;        u2 catch_type;    } exception_table[exception_table_length];    u2 attributes_count;    attribute_info attributes[attributes_count];}

The items of the Code_attribute structure are as follows:

attribute_name_index

The value of the attribute_name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info structure () representing the string "Code".

attribute_length

The value of the attribute_length item indicates the length of the attribute, excluding the initial six bytes.

max_stack

The value of the max_stack item gives the maximum depth of the operand stack of this method () at any point during execution of the method.

max_locals

The value of the max_locals item gives the number of local variables in the local variable array allocated upon invocation of this method (), including the local variables used to pass parameters to the method on its invocation.

The greatest local variable index for a value of type long or double is max_locals - 2. The greatest local variable index for a value of any other type is max_locals - 1.

code_length

The value of the code_length item gives the number of bytes in the code array for this method.

The value of code_length must be greater than zero (as the code array must not be empty) and less than 65536.

code[]

The code array gives the actual bytes of Java Virtual Machine code that implement the method.

When the code array is read into memory on a byte-addressable machine, if the first byte of the array is aligned on a 4-byte boundary, the tableswitch and lookupswitch 32-bit offsets will be 4-byte aligned. (Refer to the descriptions of those instructions for more information on the consequences of code array alignment.)

The detailed constraints on the contents of the code array are extensive and are given in a separate section ().

exception_table_length

The value of the exception_table_length item gives the number of entries in the exception_table table.

exception_table[]

Each entry in the exception_table array describes one exception handler in the code array. The order of the handlers in the exception_table array is significant ().

Each exception_table entry contains the following four items:

start_pc, end_pc

The values of the two items start_pc and end_pc indicate the ranges in the code array at which the exception handler is active. The value of start_pc must be a valid index into the code array of the opcode of an instruction. The value of end_pc either must be a valid index into the code array of the opcode of an instruction or must be equal to code_length, the length of the code array. The value of start_pc must be less than the value of end_pc.

The start_pc is inclusive and end_pc is exclusive; that is, the exception handler must be active while the program counter is within the interval [start_pc, end_pc).

The fact that end_pc is exclusive is a historical mistake in the design of the Java Virtual Machine: if the Java Virtual Machine code for a method is exactly 65535 bytes long and ends with an instruction that is 1 byte long, then that instruction cannot be protected by an exception handler. A compiler writer can work around this bug by limiting the maximum size of the generated Java Virtual Machine code for any method, instance initialization method, or static initializer (the size of any code array) to 65534 bytes.

handler_pc

The value of the handler_pc item indicates the start of the exception handler. The value of the item must be a valid index into the code array and must be the index of the opcode of an instruction.

catch_type

If the value of the catch_type item is nonzero, it must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Class_info structure () representing a class of exceptions that this exception handler is designated to catch. The exception handler will be called only if the thrown exception is an instance of the given class or one of its subclasses.

The verifier checks that the class is Throwable or a subclass of Throwable ().

If the value of the catch_type item is zero, this exception handler is called for all exceptions.

This is used to implement finally ().

attributes_count

The value of the attributes_count item indicates the number of attributes of the Code attribute.

attributes[]

Each value of the attributes table must be an attribute_info structure ().

A Code attribute can have any number of optional attributes associated with it.

The attributes defined by this specification as appearing in the attributes table of a Code attribute are listed in .

The rules concerning attributes defined to appear in the attributes table of a Code attribute are given in .

The rules concerning non-predefined attributes in the attributes table of a Code attribute are given in .

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