Java ASM系列：（035）TraceClassVisitor介绍
作者：互联网
本文属于[Java ASM系列一：Core API](https://blog.51cto.com/lsieun/2924583)当中的一篇。
`TraceClassVisitor` class extends the `ClassVisitor` class, and builds **a textual representation of the visited class**.
## 1. TraceClassVisitor类
### 1.1 class info
第一个部分，`TraceClassVisitor`类继承自`ClassVisitor`类，而且有`final`修饰，因此不会存在子类。
```java
public final class TraceClassVisitor extends ClassVisitor {
}
```
### 1.2 fields
第二个部分，`TraceClassVisitor`类定义的字段有哪些。`TraceClassVisitor`类有两个重要的字段，一个是`PrintWriter printWriter`用于打印；另一个是`Printer p`将class转换成文字信息。
```java
public final class TraceClassVisitor extends ClassVisitor {
private final PrintWriter printWriter; // 真正打印输出的类
public final Printer p; // 信息采集器
}
```
### 1.3 constructors
第三个部分，`TraceClassVisitor`类定义的构造方法有哪些。
```java
public final class TraceClassVisitor extends ClassVisitor {
public TraceClassVisitor(final PrintWriter printWriter) {
this(null, printWriter);
}
public TraceClassVisitor(final ClassVisitor classVisitor, final PrintWriter printWriter) {
this(classVisitor, new Textifier(), printWriter);
}
public TraceClassVisitor(final ClassVisitor classVisitor, final Printer printer, final PrintWriter printWriter) {
super(Opcodes.ASM10_EXPERIMENTAL, classVisitor);
this.printWriter = printWriter;
this.p = printer;
}
}
```
### 1.4 methods
第四个部分，`TraceClassVisitor`类定义的方法有哪些。对于`TraceClassVisitor`类的`visit()`、`visitField()`、`visitMethod()`和`visitEnd()`方法，会分别调用`Printer.visit()`、`Printer.visitField()`、`Printer.visitMethod()`和`Printer.visitClassEnd()`方法。
```java
public final class TraceClassVisitor extends ClassVisitor {
@Override
public void visit(final int version, final int access, final String name, final String signature,
final String superName, final String[] interfaces) {
p.visit(version, access, name, signature, superName, interfaces);
super.visit(version, access, name, signature, superName, interfaces);
}
@Override
public FieldVisitor visitField(final int access, final String name, final String descriptor,
final String signature, final Object value) {
Printer fieldPrinter = p.visitField(access, name, descriptor, signature, value);
return new TraceFieldVisitor(super.visitField(access, name, descriptor, signature, value), fieldPrinter);
}
@Override
public MethodVisitor visitMethod(final int access, final String name, final String descriptor,
final String signature, final String[] exceptions) {
Printer methodPrinter = p.visitMethod(access, name, descriptor, signature, exceptions);
return new TraceMethodVisitor(super.visitMethod(access, name, descriptor, signature, exceptions), methodPrinter);
}
@Override
public void visitEnd() {
p.visitClassEnd();
if (printWriter != null) {
p.print(printWriter); // Printer和PrintWriter进行结合
printWriter.flush();
}
super.visitEnd();
}
}
```
## 2. 如何使用TraceClassVisitor类
使用`TraceClassVisitor`类，很重点的一点就是选择`Printer`类的具体实现，可以选择`ASMifier`类，也可以选择`Textifier`类（默认）：
```text
boolean flag = true or false;
Printer printer = flag ? new ASMifier() : new Textifier();
PrintWriter printWriter = new PrintWriter(System.out, true);
TraceClassVisitor traceClassVisitor = new TraceClassVisitor(null, printer, printWriter);
```
### 2.1 生成新的类
```java
import lsieun.utils.FileUtils;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.FieldVisitor;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.util.TraceClassVisitor;
import java.io.PrintWriter;
import static org.objectweb.asm.Opcodes.*;
public class TraceClassVisitorExample01Generate {
public static void main(String[] args) throws Exception {
String relative_path = "sample/HelloWorld.class";
String filepath = FileUtils.getFilePath(relative_path);
// (1) 生成byte[]内容
byte[] bytes = dump();
// (2) 保存byte[]到文件
FileUtils.writeBytes(filepath, bytes);
}
public static byte[] dump() throws Exception {
// (1) 创建ClassWriter对象
ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
PrintWriter printWriter = new PrintWriter(System.out);
TraceClassVisitor cv = new TraceClassVisitor(cw, printWriter);
// (2) 调用visitXxx()方法
cv.visit(V1_8, ACC_PUBLIC + ACC_SUPER, "sample/HelloWorld", null, "java/lang/Object", null);
{
FieldVisitor fv1 = cv.visitField(ACC_PRIVATE, "intValue", "I", null, null);
fv1.visitEnd();
}
{
FieldVisitor fv2 = cv.visitField(ACC_PRIVATE, "strValue", "Ljava/lang/String;", null, null);
fv2.visitEnd();
}
{
MethodVisitor mv1 = cv.visitMethod(ACC_PUBLIC, "", "()V", null, null);
mv1.visitCode();
mv1.visitVarInsn(ALOAD, 0);
mv1.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "", "()V", false);
mv1.visitInsn(RETURN);
mv1.visitMaxs(0, 0);
mv1.visitEnd();
}
{
MethodVisitor mv2 = cv.visitMethod(ACC_PUBLIC, "test", "()V", null, null);
mv2.visitCode();
mv2.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv2.visitLdcInsn("Hello World");
mv2.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
mv2.visitInsn(RETURN);
mv2.visitMaxs(2, 1);
mv2.visitEnd();
}
cv.visitEnd();
// (3) 调用toByteArray()方法
return cw.toByteArray();
}
}
```
### 2.2 修改已有的类
```java
import lsieun.utils.FileUtils;
import org.objectweb.asm.Cla***eader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.util.TraceClassVisitor;
import java.io.PrintWriter;
public class TraceClassVisitorExample02Transform {
public static void main(String[] args) {
String relative_path = "sample/HelloWorld.class";
String filepath = FileUtils.getFilePath(relative_path);
byte[] bytes1 = FileUtils.readBytes(filepath);
//（1）构建Cla***eader
Cla***eader cr = new Cla***eader(bytes1);
//（2）构建ClassWriter
ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
//（3）串连ClassVisitor
int api = Opcodes.ASM9;
PrintWriter printWriter = new PrintWriter(System.out);
TraceClassVisitor tcv = new TraceClassVisitor(cw, printWriter);
ClassVisitor cv = new MethodTimerVisitor(api, tcv);
//（4）结合Cla***eader和ClassVisitor
int parsingOptions = Cla***eader.SKIP_DEBUG | Cla***eader.SKIP_FRAMES;
cr.accept(cv, parsingOptions);
//（5）生成byte[]
byte[] bytes2 = cw.toByteArray();
FileUtils.writeBytes(filepath, bytes2);
}
}
```
### 2.3 打印ASM代码
```java
import org.objectweb.asm.Cla***eader;
import org.objectweb.asm.util.ASMifier;
import org.objectweb.asm.util.Printer;
import org.objectweb.asm.util.Textifier;
import org.objectweb.asm.util.TraceClassVisitor;
import java.io.IOException;
import java.io.PrintWriter;
/**
* 这里的代码是参考自{@link org.objectweb.asm.util.Printer#main}
*/
public class ASMPrint {
public static void main(String[] args) throws IOException {
// (1) 设置参数
String className = "sample.HelloWorld";
int parsingOptions = Cla***eader.SKIP_FRAMES | Cla***eader.SKIP_DEBUG;
boolean asmCode = true;
// (2) 打印结果
Printer printer = asmCode ? new ASMifier() : new Textifier();
PrintWriter printWriter = new PrintWriter(System.out, true);
TraceClassVisitor traceClassVisitor = new TraceClassVisitor(null, printer, printWriter);
new Cla***eader(className).accept(traceClassVisitor, parsingOptions);
}
}
```
## 3. 总结
本文对`TraceClassVisitor`类进行了介绍，内容总结如下：
- 第一点，从整体上来说，`TraceClassVisitor`类的作用是什么。它能够将class文件的内容转换成文字输出。
- 第二点，从结构上来说，`TraceClassVisitor`类的各个部分包含哪些信息。
- 第三点，从使用上来说，`TraceClassVisitor`类的输出结果依赖于`Printer`类的具体实现，可以选择`ASMifier`类输出ASM代码，也可以选择`Textifier`类输出Instruction信息。
标签：printWriter,Java,String,TraceClassVisitor,new,import,final,ASM 来源： https://blog.51cto.com/lsieun/2973664