===== 6.7 Constant Pool Component =====

The Constant Pool Component contains an entry for each of the classes, methods, and fields referenced by elements in the Method Component ([[6.9_Method Component| Section 6.9, Method Component]]) of this CAP file. The referencing elements in the Method Component may be instructions in the methods or exception handler catch types in the exception handler table. 

Entries in the Constant Pool Component reference elements in the Class Component ([[6.8_Class Component| Section 6.8, Class Component]]), Method Component ([[6.9_Method Component| Section 6.9, Method Component]]), and Static Field Component ([[6.10_Static Field Component| Section 6.10, Static Field Component]]). The Import Component ([[6.6_Import Component| Section 6.6, Import Component]]) is also accessed using a package token ([[4.3.7.1_Package| Section 4.3.7.1, Package]]) to describe references to classes, methods and fields defined in imported packages. Entries in the Constant Pool Component do not reference other entries internal to itself. 

The Constant Pool Component is described by the following structure: 

The items in the **constant_pool_component** structure are as follows: 

**tag** 

The **tag** item has the value **COMPONENT_ConstantPool** (5). 

**size** 

The ******size****** item indicates the number of bytes in the **constant_pool_component** structure, excluding the **tag** and ******size****** items. The value of the ******size****** item must be greater than zero. 

**count** 

The **count** item represents the number entries in the **constant_pool[]** array. Valid values are between 0 and 65535, inclusive. 

**constant_pool[]** 

The **constant_pool[]** item represents an array of **cp_info** structures: 

Each item in the **constant_pool[]** array is a 4-byte structure. Each structure must begin with a 1-byte tag indicating the kind of **cp_info** entry. The content and format of the 3-byte info array varies with the value of the tag. The valid tags and their values are listed in the following table. 

Constant Type 

Tag 

**CONSTANT_Classref** 

**1** 

**CONSTANT_InstanceFieldref** 

**2** 

**CONSTANT_VirtualMethodref** 

**3** 

**CONSTANT_SuperMethodref** 

**4** 

**CONSTANT_StaticFieldref** 

**5** 

**CONSTANT_StaticMethodref** 

**6** 

Java Card platform constant types ("Java Card constant types") are more specific than those in Java class files. The categories indicate not only the type of the item referenced, but also the manner in which it is referenced.  

For example, in the Java constant pool there is one constant type for method references, while in the Java Card platform constant pool ("Java Card constant pool") there are three constant types for method references: one for virtual method invocations using the **invokevirtual** bytecode, one for super method invocations using the ****invokespecial**** bytecode, and one for static method invocations using either the **invokestatic** or ****invokespecial**** bytecode.<SUP>[4]</SUP> The additional information provided by a constant type in Java Card technologies simplifies resolution of references. 

There are no ordering constraints on constant pool entries. It is recommended, however, that **CONSTANT_InstanceFieldref** ([[6.7.2_**CONSTANT_InstanceFieldref**_ CONSTANT_VirtualMethodref_ and CONSTANT_SuperMethodref| Section 6.7.2, **CONSTANT_InstanceFieldref**, CONSTANT_VirtualMethodref, and CONSTANT_SuperMethodref]]) constants occur early in the array to permit using **getfield_T** and **putfield_T** bytecodes instead of **getfield_T**_w and **putfield_T**_w bytecodes. The former have 1-byte constant pool index parameters while the latter have 2-byte constant pool index parameters. 

The first entry in the constant pool can not be an exception handler class that is referenced by a ******catch_type_index****** of an ****exception_handler_info**** structure. In such a case the value of the ******catch_type_index****** would be equal to 0, but the value of 0 in a ******catch_type_index****** is reserved to indicate an ****exception_handler_info**** structure that describes a **//finally//** block.