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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"><html><head><title>VM Spec </title></head><body BGCOLOR=#eeeeff text=#000000 LINK=#0000ff VLINK=#000077 ALINK=#ff0000><table width=100%><tr><td><a href="VMSpecTOC.doc.html">Contents</a> | <a href="Instructions2.doc4.html">Prev</a> | <a href="Instructions2.doc6.html">Next</a> | <a href="VMSpecIX.fm.html">Index</a></td><td align=right><i><i>The Java<sup><font size=-2>TM</font></sup> Virtual Machine Specification</i></i></td></tr></table><hr><br> <a href="Instructions2.doc.html">A</a><a href="Instructions2.doc1.html">B</a><a href="Instructions2.doc2.html">C</a><a href="Instructions2.doc3.html">D</a><a href="Instructions2.doc4.html">F</a><a href="Instructions2.doc5.html">G</a><a href="Instructions2.doc6.html">I</a><a href="Instructions2.doc7.html">J</a><a href="Instructions2.doc8.html">L</a><a href="Instructions2.doc9.html">M</a><a href="Instructions2.doc10.html">N</a><a href="Instructions2.doc11.html">P</a><a href="Instructions2.doc12.html">R</a><a href="Instructions2.doc13.html">S</a><a href="Instructions2.doc14.html">T</a><a href="Instructions2.doc15.html">W</a><a name="getfield"></a><hr><h2>getfield</h2><a name="getfield.Operation"></a><p><b>Operation</b><br><blockquote><a name="68230"></a>Fetch field from object<p><Table Border="1"></blockquote><p><b>Format</b><br><blockquote><tr><td><a name="68225"></a> <i>getfield</i><td><a name="87568"></a> <tr><td><a name="68227"></a> <i>indexbyte1</i><td><a name="87568"></a> <tr><td><a name="68229"></a> <i>indexbyte2</i><td><a name="87568"></a> </Table><br></blockquote><p><a name="getfield.Forms"></a><p><b>Forms</b><br><blockquote><a name="68231"></a><i>getfield</i> = 180 (0xb4)</blockquote><p><a name="getfield.Operand"></a><p><b>Operand Stack</b><br><blockquote><a name="68232"></a>..., <i>objectref</i><em> </em><img src="chars/arrwdbrt.gif"> ..., <i>value</i> </blockquote><p><a name="getfield.Description"></a><p><b>Description</b><br><blockquote><a name="68235"></a>The <i>objectref</i>, which must be of type <code>reference</code>, is popped from the operand stack. The unsigned <i>indexbyte1</i> and <i>indexbyte2</i> are used to construct an index into the runtime constant pool of the current class <a href="Overview.doc.html#17257">(§3.6)</a>, where the value of the index is (<i>indexbyte1</i> << 8) | <i>indexbyte2</i>. The runtime constant pool item at that index must be a symbolic reference to a field <a href="ConstantPool.doc.html#73272">(§5.1)</a>, which gives the name and descriptor of the field as well as a symbolic reference to the class in which the field is to be found. The referenced field is resolved <a href="ConstantPool.doc.html#71685">(§5.4.3.2)</a>. The <i>value</i> of the referenced field in <i>objectref</i> is fetched and pushed onto the operand stack.</blockquote><p><blockquote><a name="278581"></a>The class of <i>objectref</i> must not be an array. If the field is <code>protected</code> <a href="ClassFile.doc.html#1513">(§4.6)</a>, and it is either a member of the current class or a member of a superclass of the current class, then the class of <i>objectref</i> must be either the current class or a subclass of the current class.</blockquote><p><a name="getfield.Linking"></a><p><b>Linking Exceptions</b><br><blockquote><a name="346057"></a>During resolution of the symbolic reference to the field, any of the errors pertaining to field resolution documented in <a href="ConstantPool.doc.html#71685">Section 5.4.3.2</a> can be thrown.</blockquote><p><blockquote><a name="346062"></a>Otherwise, if the resolved field is a <code>static</code> field, <i>getfield</i> throws an <code>IncompatibleClassChangeError</code>.</blockquote><p><blockquote><a name="346054"></a></blockquote><p><a name="getfield.Runtime"></a><p><b>Runtime Exception</b><br><blockquote><a name="68249"></a>Otherwise, if <i>objectref</i><em> </em>is <code>null</code>, the <i>getfield</i> instruction throws a <code>NullPointerException</code>.</blockquote><p><a name="getfield.Notes"></a><p><b>Notes</b><br><blockquote><a name="203076"></a>The <i>getfield</i> instruction cannot be used to access the <code>length</code> field of an array. The <i>arraylength </i>instruction is used instead.</blockquote><p><a name="getstatic"></a><hr><h2>getstatic</h2><a name="getstatic.Operation"></a><p><b>Operation</b><br><blockquote><a name="68261"></a>Get <code>static</code> field from class<p><Table Border="1"></blockquote><p><b>Format</b><br><blockquote><tr><td><a name="68256"></a> <i>getstatic</i><td><a name="87568"></a> <tr><td><a name="68258"></a> <i>indexbyte1</i><td><a name="87568"></a> <tr><td><a name="68260"></a> <i>indexbyte2</i><td><a name="87568"></a> </Table><br></blockquote><p><a name="getstatic.Forms"></a><p><b>Forms</b><br><blockquote><a name="68262"></a><i>getstatic</i> = 178 (0xb2)</blockquote><p><a name="getstatic.Operand"></a><p><b>Operand Stack</b><br><blockquote><a name="68263"></a>...,<em> </em><img src="chars/arrwdbrt.gif"> ..., <i>value</i> </blockquote><p><a name="getstatic.Description"></a><p><b>Description</b><br><blockquote><a name="68266"></a>The unsigned <i>indexbyte1</i> and <i>indexbyte2</i> are used to construct an index into the runtime constant pool of the current class <a href="Overview.doc.html#17257">(§3.6)</a>, where the value of the index is (<i>indexbyte1</i> << 8) | <i>indexbyte2</i>. The runtime constant pool item at that index must be a symbolic reference to a field <a href="ConstantPool.doc.html#73272">(§5.1)</a>, which gives the name and descriptor of the field as well as a symbolic reference to the class or interface in which the field is to be found. The referenced field is resolved <a href="ConstantPool.doc.html#71685">(§5.4.3.2)</a>. </blockquote><p><blockquote><a name="280902"></a>On successful resolution of the field, the class or interface that declared the resolved field is initialized <a href="ConstantPool.doc.html#77976">(§5.5)</a> if that class or interface has not already been initialized.</blockquote><p><blockquote><a name="280899"></a>The <i>value</i> of the class or interface field is fetched and pushed onto the operand stack.</blockquote><p><a name="getstatic.Linking"></a><p><b>Linking Exceptions</b><br><blockquote><a name="346080"></a>During resolution of the symbolic reference to the class or interface field, any of the exceptions pertaining to field resolution documented in <a href="ConstantPool.doc.html#71685">Section 5.4.3.2</a> can be thrown.</blockquote><p><blockquote><a name="346084"></a>Otherwise, if the resolved field is not a <code>static</code> (class) field or an interface field, <i>getstatic</i> throws an <code>IncompatibleClassChangeError</code>.</blockquote><p><a name="getstatic.Runtime"></a><p><b>Runtime Exception</b><br><blockquote><a name="333038"></a>Otherwise, if execution of this <i>getstatic</i> instruction causes initialization of the referenced class or interface, <i>getstatic</i> may throw an <code>Error</code> as detailed in <a href="Concepts.doc.html#24237">Section 2.17.5</a>.</blockquote><p><a name="goto"></a><hr><h2>goto</h2><a name="goto.Operation"></a><p><b>Operation</b><br><blockquote><a name="68293"></a>Branch always<p><Table Border="1"></blockquote><p><b>Format</b><br><blockquote><tr><td><a name="68288"></a> <i>goto</i><td><a name="87568"></a> <tr><td><a name="68290"></a> <i>branchbyte1</i><td><a name="87568"></a> <tr><td><a name="68292"></a> <i>branchbyte2</i><td><a name="87568"></a> </Table><br></blockquote><p><a name="goto.Forms"></a><p><b>Forms</b><br><blockquote><a name="68294"></a><i>goto</i> = 167 (0xa7)</blockquote><p><a name="goto.Operand"></a><p><b>Operand Stack</b><br><blockquote><a name="68295"></a>No change</blockquote><p><a name="goto.Description"></a><p><b>Description</b><br><blockquote><a name="68296"></a>The unsigned bytes <i>branchbyte1</i> and <i>branchbyte2</i> are used to construct a signed 16-bit <i>branchoffset</i>, where <i>branchoffset</i> is (<i>branchbyte1</i> << 8) | <i>branchbyte2</i>. Execution proceeds at that offset from the address of the opcode of this <i>goto</i> instruction. The target address must be that of an opcode of an instruction within the method that contains this <i>goto</i> instruction.</blockquote><p><a name="goto_w"></a><hr><h2>goto_w</h2><a name="goto_w.Operation"></a><p><b>Operation</b><br><blockquote><a name="68311"></a>Branch always (wide index)<p><Table Border="1"></blockquote><p><b>Format</b><br><blockquote><tr><td><a name="68302"></a> <i>goto_w</i><td><a name="87568"></a> <tr><td><a name="68304"></a> <i>branchbyte1</i><td><a name="87568"></a> <tr><td><a name="68306"></a> <i>branchbyte2</i><td><a name="87568"></a> <tr><td><a name="68308"></a> <i>branchbyte3</i><td><a name="87568"></a> <tr><td><a name="68310"></a> <i>branchbyte4</i><td><a name="87568"></a> </Table><br></blockquote><p><a name="goto_w.Forms"></a><p><b>Forms</b><br><blockquote><a name="68312"></a><i>goto_w</i> = 200 (0xc8)</blockquote><p><a name="goto_w.Operand"></a><p><b>Operand Stack</b><br><blockquote><a name="68313"></a>No change</blockquote><p><a name="goto_w.Description"></a><p><b>Description</b><br><blockquote><a name="68314"></a>The unsigned bytes <i>branchbyte1</i>, <i>branchbyte2</i>, <i>branchbyte3</i>, and <i>branchbyte4</i> are used to construct a signed 32-bit <i>branchoffset</i>, where <i>branchoffset</i> is (<i>branchbyte1</i> << 24) | (<i>branchbyte2</i> << 16) | (<i>branchbyte3</i> << 8) | <i>branchbyte4</i>. Execution proceeds at that offset from the address of the opcode of this <i>goto_w</i> instruction. The target address must be that of an opcode of an instruction within the method that contains this <i>goto_w</i> instruction.</blockquote><p><a name="goto_w.Notes"></a><p><b>Notes</b><br><blockquote><a name="68315"></a>Although the <i>goto_w</i> instruction takes a 4-byte branch offset, other factors limit the size of a method to 65535 bytes <a href="ClassFile.doc.html#88659">(§4.10)</a>. This limit may be raised in a future release of the Java virtual machine.</blockquote><p><hr><!-- This inserts footnotes--><p><a href="VMSpecTOC.doc.html">Contents</a> | <a href="Instructions2.doc4.html">Prev</a> | <a href="Instructions2.doc6.html">Next</a> | <a href="VMSpecIX.fm.html">Index</a><p><font size="-1"><i>The Java</i><sup><font size=-2>TM</font></sup><i> Virtual Machine Specification </i><br><!-- HTML generated by Suzette Pelouch on March 30, 1999 --><i><a href="Copyright.doc.html">Copyright © 1999 Sun Microsystems, Inc.</a>All rights reserved</i><br>Please send any comments or corrections to <a href="mailto:jvm@java.sun.com">jvm@java.sun.com</a></font></body></html>⌨️ 快捷键说明
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