Java 对象的list转换成json java将list转为json

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mob64ca1401464d 2024-06-26 15:26:15

文章标签 Java 对象的list转换成json java list转json报错 java json JSON 文章分类 Java 后端开发

今天给一个java类加了几个字段，没想到转为json字符串的时候报错了，定位了一下原因，觉得这种情况遇到的应该不多，又想起来很久没写博客了，于是就把遇到的问题以及定位过程记下来，省了以后遇到的人再花时间定位了

调用的是JSON.toJSONString(Object object)方法，具体的报错信息如下：Caused by: com.alibaba.fastjson.JSONException: write javaBean error
at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:234)
at Serializer_5.write1(Unknown Source)
at Serializer_5.write(Unknown Source)
at com.alibaba.fastjson.serializer.JSONSerializer.write(JSONSerializer.java:369)
at com.alibaba.fastjson.JSON.toJSONString(JSON.java:393)
at com.alibaba.fastjson.JSON.toJSONString(JSON.java:567)
at net.sf.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204)
at org.springframework.aop.framework.Cglib2AopProxy$CglibMethodInvocation.invokeJoinpoint(Cglib2AopProxy.java:689)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150)
at org.springframework.aop.aspectj.AspectJAfterThrowingAdvice.invoke(AspectJAfterThrowingAdvice.java:55)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
at
... 81 common frames omitted
Caused by: com.alibaba.fastjson.JSONException: create asm serializer error, class class com.feifei.bean.TestBean
at com.alibaba.fastjson.serializer.SerializeConfig.createJavaBeanSerializer(SerializeConfig.java:113)
at com.alibaba.fastjson.serializer.JSONSerializer.getObjectWriter(JSONSerializer.java:527)
at com.alibaba.fastjson.serializer.ListSerializer.write(ListSerializer.java:81)
at com.alibaba.fastjson.serializer.ObjectFieldSerializer.writeValue(ObjectFieldSerializer.java:118)
at com.alibaba.fastjson.serializer.ObjectFieldSerializer.writeProperty(ObjectFieldSerializer.java:67)
at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:216)
... 94 common frames omitted
Caused by: java.lang.ClassFormatError: JVMCFRE042 bytecode array size > 65535; class=Serializer_57, offset=53261
at java.lang.ClassLoader.defineClassImpl(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:275)
at com.alibaba.fastjson.util.ASMClassLoader.defineClassPublic(ASMClassLoader.java:42)
at com.alibaba.fastjson.serializer.ASMSerializerFactory.createJavaBeanSerializer(ASMSerializerFactory.java:287)
at com.alibaba.fastjson.serializer.ASMSerializerFactory.createJavaBeanSerializer(ASMSerializerFactory.java:36)
at com.alibaba.fastjson.serializer.SerializeConfig.createASMSerializer(SerializeConfig.java:78)
at com.alibaba.fastjson.serializer.SerializeConfig.createJavaBeanSerializer(SerializeConfig.java:106)
... 99 common frames omitted
可以看到，代码在执行到ASMClassLoader.defineClassPublic这个方法时报错了，报的错是：JVMCFRE042 bytecode array size > 65535
字节数组超长,这个长度65535，很显然是长度规定了2的16次方(65536)，也就是64K大小。
再来看看ASMClassLoader这个类，这个类继承了java的类装载器ClassLoader，fastjson内嵌了ASM框架来动态生成类；public class ASMClassLoader extends ClassLoader {
并且报错的方法其实就是调用了ClassLoader的类定义方法defineClass，本质上调用的是JVM的native功能public Class> defineClassPublic(String name, byte[] b, int off, int len) throws ClassFormatError {
Class> clazz = defineClass(name, b, off, len, DOMAIN);
return clazz;
}
这里其实也就是传的需要生成的类的字节码，在这里也就对应着我们需要转换成JSON串的对象类的字节码，所以这个报错应该就是对象类字节码超长了,也就是类里面的东西太多了,试着删掉这个bean里的一些变量，测试一把，果然不报错了。
那到底是什么太长了呢？这也好办，继续写个单元测试，把ASMSerializerFactory类拷出来，把字节码输出看看
单元测试：@Test
public void test(){
try{
this.asmSerial(TestBean.class);
}catch(Exception e){
System.out.println(e);
}
}
private void asmSerial(Class> clazz) throws Exception{
ASMSerializerFactory fac = new ASMSerializerFactory();
fac.createJavaBeanSerializer(clazz);
}
在拷出来的ASMSerializerFactory类的createJavaBeanSerializer方法中新增输出逻辑：byte[] code = cw.toByteArray();
// 输出class文件
File outputFile = new File("C:\\Users\\feifei\\Desktop\\ooo\\out.class");
FileOutputStream outputFileStream = null;
try {
outputFileStream = new FileOutputStream(outputFile);
} catch (FileNotFoundException e) {
}
outputFileStream.write(code);
System.out.println("code:length: " + code.length);
执行一把，果然报错了，而且class文件也成功生成了，先看看控制台输出：[TestNG] Running:
C:\Users\feifei\AppData\Local\Temp\testng-eclipse-1963807168\testng-customsuite.xml
code:length: 191423
FAILED: test
java.lang.ClassFormatError: Invalid method Code length 65904 in class file Serializer_1
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at com.alibaba.fastjson.util.ASMClassLoader.defineClassPublic(ASMClassLoader.java:42)
at ASMSerializerFactory.createJavaBeanSerializer(ASMSerializerFactory.java:299)
at ASMSerializerFactory.createJavaBeanSerializer(ASMSerializerFactory.java:42)
at tt.asmSerial(tt.java:19)
at tt.test(tt.java:11)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
可见报错更加细化了java.lang.ClassFormatError: Invalid method Code length 65904 in class file Serializer_1

这个报错意思更加明显了，方法的长度不合法，这个方法的长度是65904，结合上面报出来的大于65535，那么问题就很明显了，java里规定了类的一个方法最大不能超过64K。

我们继续打开刚刚生成的class文件，可以看到，我们bean里面的字段都在里面，而里面最大的方法是write和write1，分别占13000多行，所以应该就是这两方法超长了。

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