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A Java virtual machine instruction consists of an opcode specifying the operation to be performed, f

<td><a name="88367"></a>
<code>0x0001</code>
<td><a name="88369"></a>
Declared <code>public</code>; may be accessed from outside its package. 

<tr><td><a name="88373"></a>
<code>ACC_PRIVATE</code>
<td><a name="88375"></a>
<code>0x0002</code>
<td><a name="88377"></a>
Declared <code>private</code>; usable only within the defining class.

<tr><td><a name="88381"></a>
<code>ACC_PROTECTED</code>
<td><a name="88383"></a>
<code>0x0004</code>
<td><a name="88385"></a>
Declared <code>protected</code>; may be accessed within subclasses.

<tr><td><a name="88389"></a>
<code>ACC_STATIC</code>
<td><a name="88391"></a>
<code>0x0008</code>
<td><a name="88393"></a>
Declared <code>static</code>. 

<tr><td><a name="88396"></a>
<code>ACC_FINAL</code>
<td><a name="88398"></a>
<code>0x0010</code>
<td><a name="88400"></a>
Declared <code>final</code>; no further assignment after initialization.

<tr><td><a name="88404"></a>
<code>ACC_VOLATILE</code>
<td><a name="88406"></a>
<code>0x0040</code>
<td><a name="88408"></a>
Declared <code>volatile</code>; cannot be cached. 

<tr><td><a name="88412"></a>
<code>ACC_TRANSIENT</code>
<td><a name="88414"></a>
<code>0x0080</code>
<td><a name="88416"></a>
Declared <code>transient</code>; not written or read by a persistent object manager. 

</Table><br><br><p>
Fields of classes may set any of the flags in <a href="ClassFile.doc.html#88358">Table 4.4</a>. However, a specific field of a class may have at most one of its <code>ACC_PRIVATE</code>, <code>ACC_PROTECTED</code>, and <code>ACC_PUBLIC</code> flags set <a href="Concepts.doc.html#18914">(&#167;2.7.4)</a> and may not have both its <code>ACC_FINAL</code> and <code>ACC_VOLATILE</code> flags set <a href="Concepts.doc.html#29882">(&#167;2.9.1)</a>.<p>

All fields of interfaces must have their <code>ACC_PUBLIC</code>, <code>ACC_STATIC</code>, and <code>ACC_FINAL</code> flags set and may not have any of the other flags in <a href="ClassFile.doc.html#88358">Table 4.4</a> set <a href="Concepts.doc.html#18349">(&#167;2.13.3.1)</a>. <p>

All bits of the <code>access_flags</code> item not assigned in <a href="ClassFile.doc.html#88358">Table 4.4</a> are reserved for future use. They should be set to zero in generated <code>class</code> files and should be ignored by Java virtual machine implementations.<p>
<a name="22218"></a>
<dt><code>name_index</code>
<dd> The value of the <code>name_index</code> item must be a valid index into the <code>constant_pool</code> table. The <code>constant_pool</code> entry at that index must be a <code>CONSTANT_Utf8_info</code> <a href="ClassFile.doc.html#7963">(&#167;4.4.7)</a> structure which must represent a valid field name <a href="Concepts.doc.html#21272">(&#167;2.7)</a> stored as a simple name <a href="Concepts.doc.html#21410">(&#167;2.7.1)</a>, that is, as a Java programming language identifier <a href="Concepts.doc.html#25339">(&#167;2.2)</a>.<p>
<a name="1497"></a>
<dt><code>descriptor_index</code>
<dd> The value of the <code>descriptor_index</code> item must be a valid index into the <code>constant_pool</code> table. The <code>constant_pool</code> entry at that index must be a <code>CONSTANT_Utf8_info</code> <a href="ClassFile.doc.html#7963">(&#167;4.4.7)</a> structure that must represent a valid field descriptor <a href="ClassFile.doc.html#14152">(&#167;4.3.2)</a>.<p>
<a name="1499"></a>
<dt><code>attributes_count</code>
<dd> The value of the <code>attributes_count</code> item indicates the number of additional attributes <a href="ClassFile.doc.html#43817">(&#167;4.7)</a> of this field.<p>
<a name="1501"></a>
<dt><code>attributes[]</code>
<dd> Each value of the <code>attributes</code> table must be an attribute structure <a href="ClassFile.doc.html#43817">(&#167;4.7)</a>. A field can have any number of attributes associated with it. <p>
The attributes defined by this specification as appearing in the <code>attributes</code> table of a <code>field_info</code> structure are the <code>ConstantValue</code> <a href="ClassFile.doc.html#1405">(&#167;4.7.2)</a>, <code>Synthetic</code> <a href="ClassFile.doc.html#80128">(&#167;4.7.6)</a>, and <code>Deprecated</code> <a href="ClassFile.doc.html#78232">(&#167;4.7.10)</a> attributes. <p>

A Java virtual machine implementation must recognize and correctly read <code>ConstantValue</code> <a href="ClassFile.doc.html#1405">(&#167;4.7.2)</a> attributes found in the <code>attributes</code> table of a <code>field_info</code> structure. A Java virtual machine implementation is required to silently ignore any or all other attributes in the <code>attributes</code> table that it does not recognize. Attributes not defined in this specification are not allowed to affect the semantics of the <code>class</code> file, but only to provide additional descriptive information <a href="ClassFile.doc.html#16733">(&#167;4.7.1)</a>.<p>
<a name="1513"></a>
</dl>
<hr><h2>4.6    Methods</h2>
<a name="1514"></a>
Each method, including each instance initialization method <a href="Overview.doc.html#12174">(&#167;3.9)</a> and the class or 
interface initialization method <a href="Overview.doc.html#12174">(&#167;3.9)</a>, is described by a <code>method_info</code> structure. No 
two methods in one <code>class</code> file may have the same name and descriptor <a href="ClassFile.doc.html#7035">(&#167;4.3.3)</a>.
<p><a name="84874"></a>
The structure has the following format:<p>
<pre><br><a name="1515"></a>&nbsp;&nbsp;&nbsp;&nbsp;<code>method_info {
</code>&nbsp;&nbsp;&nbsp;&nbsp;<code>	u2 access_flags;
</code>&nbsp;&nbsp;&nbsp;&nbsp;<code>	u2 name_index;
</code>&nbsp;&nbsp;&nbsp;&nbsp;<code>	u2 descriptor_index;
</code>&nbsp;&nbsp;&nbsp;&nbsp;<code>	u2 attributes_count;
</code>&nbsp;&nbsp;&nbsp;&nbsp;<code>	attribute_info attributes[attributes_count];
</code><a name="1521"></a>&nbsp;&nbsp;&nbsp;&nbsp;<code>}
</code><br></pre><a name="9412"></a>
The items of the <code>method_info</code> structure are as follows:
<p><a name="1522"></a>
<dl><dt><code>access_flags</code>&#32;
<dd> The value of the <code>access_flags</code> item is a mask of flags used to denote access permission to and properties of this method. The interpretation of each flag, when set, is as shown in <a href="ClassFile.doc.html#75568">Table 4.5</a>.
<a name="75568"></a><p><Table Border="1">
<tr><td><a name="75497"></a>
<strong>Flag Name</strong>
<td><a name="75499"></a>
<strong>Value</strong>
<td><a name="75501"></a>
<strong>Interpretation</strong>

<tr><td><a name="75504"></a>
<code>ACC_PUBLIC</code>
<td><a name="75506"></a>
<code>0x0001</code>
<td><a name="75508"></a>
Declared <code>public</code>; may be accessed from outside its package. 

<tr><td><a name="75511"></a>
<code>ACC_PRIVATE</code>
<td><a name="75513"></a>
<code>0x0002</code>
<td><a name="75515"></a>
Declared <code>private</code>; accessible only within the defining class.

<tr><td><a name="75518"></a>
<code>ACC_PROTECTED</code>
<td><a name="75520"></a>
<code>0x0004</code>
<td><a name="75522"></a>
Declared <code>protected</code>; may be accessed within subclasses.

<tr><td><a name="75525"></a>
<code>ACC_STATIC</code>
<td><a name="75527"></a>
<code>0x0008</code>
<td><a name="75529"></a>
Declared <code>static</code>. 

<tr><td><a name="75533"></a>
<code>ACC_FINAL</code>
<td><a name="75535"></a>
<code>0x0010</code>
<td><a name="75537"></a>
Declared <code>final</code>; may not be overridden.

<tr><td><a name="75541"></a>
<code>ACC_SYNCHRONIZED</code>
<td><a name="75543"></a>
<code>0x0020</code>
<td><a name="75545"></a>
Declared <code>synchronized</code>; invocation is wrapped in a monitor lock.

<tr><td><a name="75549"></a>
<code>ACC_NATIVE</code>
<td><a name="75551"></a>
<code>0x0100</code>
<td><a name="75553"></a>
Declared <code>native</code>; implemented in a language other than Java.

<tr><td><a name="75555"></a>
<code>ACC_ABSTRACT</code>
<td><a name="75557"></a>
<code>0x0400</code>
<td><a name="75559"></a>
Declared <code>abstract</code>; no implementation is provided. 

<tr><td><a name="75561"></a>
<code>ACC_STRICT</code>
<td><a name="75563"></a>
<code>0x0800</code>
<td><a name="75565"></a>
Declared <code>strictfp</code>; floating-point mode is FP-strict

</Table><br><br> <p>
Methods of classes may set any of the flags in <a href="ClassFile.doc.html#75568">Table 4.5</a>. However, a specific method of a class may have at most one of its <code>ACC_PRIVATE</code>, <code>ACC_PROTECTED</code>, and <code>ACC_PUBLIC </code>flags set <a href="Concepts.doc.html#18914">(&#167;2.7.4)</a>. If such a method has its <code>ACC_ABSTRACT</code> flag set it may not have any of its <code>ACC_FINAL</code>, <code>ACC_NATIVE</code>, <code>ACC_PRIVATE</code>, <code>ACC_STATIC</code>, <code>ACC_STRICT</code>, or <code>ACC_SYNCHRONIZED</code> flags set <a href="Concepts.doc.html#20648">(&#167;2.13.3.2)</a>.<p>

All interface methods must have their <code>ACC_ABSTRACT</code> and <code>ACC_PUBLIC</code> flags set and may not have any of the other flags in <a href="ClassFile.doc.html#75568">Table 4.5</a> set <a href="Concepts.doc.html#20648">(&#167;2.13.3.2)</a>.<p>

A specific instance initialization method <a href="Overview.doc.html#12174">(&#167;3.9)</a> may have at most one of its <code>ACC_PRIVATE</code>, <code>ACC_PROTECTED</code>, and <code>ACC_PUBLIC</code> flags set and may also have its <code>ACC_STRICT</code> flag set, but may not have any of the other flags in <a href="ClassFile.doc.html#75568">Table 4.5</a> set.<p>

Class and interface initialization methods <a href="Overview.doc.html#12174">(&#167;3.9)</a> are called implicitly by the Java virtual machine; the value of their <code>access_flags</code> item is ignored except for the settings of the <code>ACC_STRICT </code>flag.<p>

All bits of the <code>access_flags</code> item not assigned in <a href="ClassFile.doc.html#75568">Table 4.5</a> are reserved for future use. They should be set to zero in generated <code>class</code> files and should be ignored by Java virtual machine implementations.<p>
<a name="1529"></a>
<dt><code>name_index</code>
<dd> The value of the <code>name_index</code> item must be a valid index into the <code>constant_pool</code> table. The <code>constant_pool</code> entry at that index must be a <code>CONSTANT_Utf8_info</code> <a href="ClassFile.doc.html#7963">(&#167;4.4.7)</a> structure representing either one of the special method names <a href="Overview.doc.html#12174">(&#167;3.9)</a>, <code>&lt;init&gt;</code> or <code>&lt;clinit&gt;</code>, or a valid method name in the Java programming language <a href="Concepts.doc.html#21272">(&#167;2.7)</a>, stored as a simple name <a href="Concepts.doc.html#21410">(&#167;2.7.1)</a>.<p>
<a name="1531"></a>
<dt><code>descriptor_index</code>
<dd> The value of the <code>descriptor_index</code> item must be a valid index into the <code>constant_pool</code> table. The <code>constant_pool</code> entry at that index must be a <code>CONSTANT_Utf8_info</code> <a href="ClassFile.doc.html#7963">(&#167;4.4.7)</a> structure representing a valid method descriptor <a href="ClassFile.doc.html#7035">(&#167;4.3.3)</a>.<p>
<a name="1533"></a>
<dt><code>attributes_count</code>
<dd> The value of the <code>attributes_count</code> item indicates the number of additional attributes <a href="ClassFile.doc.html#43817">(&#167;4.7)</a> of this method.<p>
<a name="1535"></a>
<dt><code>attributes[]</code>
<dd> Each value of the <code>attributes</code> table must be an attribute structure <a href="ClassFile.doc.html#43817">(&#167;4.7)</a>. A method can have any number of optional attributes associated with it. <p>

The only attributes defined by this specification as appearing in the <code>attributes</code> table of a <code>method_info</code> structure are the <code>Code</code> <a href="ClassFile.doc.html#1546">(&#167;4.7.3)</a>, <code>Exceptions</code> <a href="ClassFile.doc.html#3129">(&#167;4.7.4)</a>, <code>Synthetic</code> <a href="ClassFile.doc.html#80128">(&#167;4.7.6)</a>, and <code>Deprecated</code> <a href="ClassFile.doc.html#78232">(&#167;4.7.10)</a> attributes. <p>

A Java virtual machine implementation must recognize and correctly read <code>Code</code> <a href="ClassFile.doc.html#1546">(&#167;4.7.3)</a> and <code>Exceptions</code> <a href="ClassFile.doc.html#3129">(&#167;4.7.4)</a> attributes found in the <code>attributes</code> table of a <code>method_info</code> structure. A Java virtual machine implementation is required to silently ignore any or all other attributes in the <code>attributes</code> table of a <code>method_info</code> structure that it does not recognize. Attributes not defined in this specification are not allowed to affect the semantics of the <code>class</code> file, but only to provide additional descriptive information <a href="ClassFile.doc.html#16733">(&#167;4.7.1)</a>.<p>
</dl>
<a name="43817"></a>
<hr><h2>4.7    Attributes</h2>
<a name="43824"></a>
Attributes are used in the <code>ClassFile</code> <a href="ClassFile.doc.html#74353">(&#167;4.1)</a>, <code>field_info</code> <a href="ClassFile.doc.html#2877">(&#167;4.5)</a>, <code>method_info</code> 
<a href="ClassFile.doc.html#1513">(&#167;4.6)</a>, and <code>Code_attribute</code> <a href="ClassFile.doc.html#1546">(&#167;4.7.3)</a> structures of the <code>class</code> file format. All 
attributes have the following general format:
<p><pre><br><a name="1712"></a>&nbsp;&nbsp;&nbsp;&nbsp;<code>attribute_info {
</code>&nbsp;&nbsp;&nbsp;&nbsp;<code>	u2 attribute_name_index;
</code>&nbsp;&nbsp;&nbsp;&nbsp;<code>	u4 attribute_length;
</code>&nbsp;&nbsp;&nbsp;&nbsp;<code>	u1 info[attribute_length];
</code><a name="1716"></a>&nbsp;&nbsp;&nbsp;&nbsp;<code>}
</code><br></pre><a name="42211"></a>
For all attributes, the <code>attribute_name_index</code> must be a valid unsigned 16-bit index into the constant pool of the class. The <code>constant_pool</code> entry at <code>attribute_name_index</code> must be a <code>CONSTANT_Utf8_info</code> <a href="ClassFile.doc.html#7963">(&#167;4.4.7)</a> structure representing the name of the attribute. The value of the <code>attribute_length</code> item indicates the length of the subsequent information in bytes. The length does not include the initial six bytes that contain the <code>attribute_name_index</code> and <code>attribute_length</code> items.<p>
<a name="14354"></a>
Certain attributes are predefined as part of the <code>class</code> file specification. The predefined attributes are