FST序列化/反序列化

FST序列化/反序列化

一、FST序列化存储格式

基本上所有以Byte形式存储的序列化对象都是类似的存储结构，不管class文件、so文件、dex文件都是类似，可以看看这个，这方面没有什么创新的格式，最多是在字段内容上做了一些压缩优化，包括我们最常使用的utf-8编码都是这个做法。

FST的序列化存储和一般的字节格式化存储方案也没有标新立异的地方，比如下面这个FTS的序列化字节文件

00000001:  0001 0f63 6f6d 2e66 7374 2e46 5354 4265
00000010:  616e f701 fc05 7630 7374 7200 

格式：Header|类名长度|类名String|字段1类型(1Byte) | [长度] | 内容|字段2类型(1Byte) | [长度] | 内容|…

0000：字节数组类型：00标识OBJECT
0001：类名编码，00标识UTF编码，01表示ASCII编码
0002：Length of class name (1Byte) = 15
0003~0011：Class name string (15Byte)
0012：Integer类型标识 0xf7
0013：Integer的值=1
0014：String类型标识 0xfc
0015：String的长度=5
0016~001a：String的值"v0str"
001b~001c：END

从上面可以看到Integer类型序列化后只占用了一个字节（值等于1），并不像在内存中占用4Byte，所以可以看出是根据一定规则做了压缩，具体代码看FSTObjectInput#instantiateSpecialTag中对不同类型的读取，FSTObjectInput也定义不同类型对应的枚举值：

public class FSTObjectOutput implements ObjectOutput {

    private static final FSTLogger LOGGER = FSTLogger.getLogger(FSTObjectOutput.class);
    public static Object NULL_PLACEHOLDER = new Object() { public String toString() { return "NULL_PLACEHOLDER"; }};
    public static final byte SPECIAL_COMPATIBILITY_OBJECT_TAG = -19; // see issue 52
    public static final byte ONE_OF = -18;
    public static final byte BIG_BOOLEAN_FALSE = -17;
    public static final byte BIG_BOOLEAN_TRUE = -16;
    public static final byte BIG_LONG = -10;
    public static final byte BIG_INT = -9;
    public static final byte DIRECT_ARRAY_OBJECT = -8;
    public static final byte HANDLE = -7;
    public static final byte ENUM = -6;
    public static final byte ARRAY = -5;
    public static final byte STRING = -4;
    public static final byte TYPED = -3; // var class == object written class
    public static final byte DIRECT_OBJECT = -2;
    public static final byte NULL = -1;
    public static final byte OBJECT = 0;
    protected FSTEncoder codec;
    ...
}

FST序列化和反序列化原理

对Object进行Byte序列化，相当于做了持久化的存储，在反序列的时候，如果Bean的定义发生了改变，那么反序列化器就要做兼容的解决方案，我们知道对于JDK的序列化和反序列，serialVersionUID对版本控制起了很重要的作用。FTS对这个问题的解决方案是通过@Version注解进行排序。

在进行反序列操作的时候，FST会先反射或者对象Class的所有成员，并对这些成员进行了排序，请注意这里被标红的排序，因为这个排序对兼容起了关键作用，也就是@Version的原理。在FSTClazzInfo中定义了一个defFieldComparator比较器，用于对Bean的所有Field进行排序：

public final class FSTClazzInfo {
    public static final Comparator<FSTFieldInfo> defFieldComparator = new Comparator<FSTFieldInfo>() {
        @Override
        public int compare(FSTFieldInfo o1, FSTFieldInfo o2) {
            int res = 0;

            if ( o1.getVersion() != o2.getVersion() ) {
                return o1.getVersion() < o2.getVersion() ? -1 : 1;
            }

            // order: version, boolean, primitives, conditionals, object references
            if (o1.getType() == boolean.class && o2.getType() != boolean.class) {
                return -1;
            }
            if (o1.getType() != boolean.class && o2.getType() == boolean.class) {
                return 1;
            }

            if (o1.isConditional() && !o2.isConditional()) {
                res = 1;
            } else if (!o1.isConditional() && o2.isConditional()) {
                res = -1;
            } else if (o1.isPrimitive() && !o2.isPrimitive()) {
                res = -1;
            } else if (!o1.isPrimitive() && o2.isPrimitive())
                res = 1;
//                if (res == 0) // 64 bit / 32 bit issues
//                    res = (int) (o1.getMemOffset() - o2.getMemOffset());
            if (res == 0)
                res = o1.getType().getSimpleName().compareTo(o2.getType().getSimpleName());
            if (res == 0)
                res = o1.getName().compareTo(o2.getName());
            if (res == 0) {
                return o1.getField().getDeclaringClass().getName().compareTo(o2.getField().getDeclaringClass().getName());
            }
            return res;
        }
    };
    ...
}

从代码实现上可以看到，比较的优先级是Field的Version大小，然后是Field类型，所以总的来说Version越大排序越靠后，至于为什么要排序，看下FSTObjectInput#instantiateAndReadNoSer方法

public class FSTObjectInput implements ObjectInput {
	protected Object instantiateAndReadNoSer(Class c, FSTClazzInfo clzSerInfo, FSTClazzInfo.FSTFieldInfo referencee, int readPos) throws Exception {
        Object newObj;
        newObj = clzSerInfo.newInstance(getCodec().isMapBased());
        ...
        } else {
            FSTClazzInfo.FSTFieldInfo[] fieldInfo = clzSerInfo.getFieldInfo();
            readObjectFields(referencee, clzSerInfo, fieldInfo, newObj,0,0);
        }
        return newObj;
    }

    protected void readObjectFields(FSTClazzInfo.FSTFieldInfo referencee, FSTClazzInfo serializationInfo, FSTClazzInfo.FSTFieldInfo[] fieldInfo, Object newObj, int startIndex, int version) throws Exception {
        
        if ( getCodec().isMapBased() ) {
            readFieldsMapBased(referencee, serializationInfo, newObj);
            if ( version >= 0 && newObj instanceof Unknown == false)
                getCodec().readObjectEnd();
            return;
        }
        if ( version < 0 )
            version = 0;
        int booleanMask = 0;
        int boolcount = 8;
        final int length = fieldInfo.length;
        int conditional = 0;
        for (int i = startIndex; i < length; i++) {	// 注意这里的循环
            try {
                FSTClazzInfo.FSTFieldInfo subInfo = fieldInfo[i];
                if (subInfo.getVersion() > version ) {	 // 需要进入下一个版本的迭代
                    int nextVersion = getCodec().readVersionTag();	// 对象流的下一个版本
                    if ( nextVersion == 0 ) // old object read
                    {
                        oldVersionRead(newObj);
                        return;
                    }
                    if ( nextVersion != subInfo.getVersion() ) {	// 同一个Field的版本不允许变，并且版本变更和流的版本保持同步
                        throw new RuntimeException("read version tag "+nextVersion+" fieldInfo has "+subInfo.getVersion());
                    }
					readObjectFields(referencee,serializationInfo,fieldInfo,newObj,i,nextVersion);	// 开始下一个Version的递归
                    return;
                }
                if (subInfo.isPrimitive()) {
                	...
                } else {
                    if ( subInfo.isConditional() ) {
                    	...
                    }
                   	// object 把读出来的值保存到FSTFieldInfo中
                    Object subObject = readObjectWithHeader(subInfo);
                    subInfo.setObjectValue(newObj, subObject);
				}
				...

从这段代码的逻辑基本就可以知道FST的序列化和反序列化兼容的原理了，注意里面的循环，正是按照排序后的Filed进行循环，而每个FSTFieldInfo都记录自己在对象流中的位置、类型等详细信息：

序列化：

1. 按照Version对Bean的所有Field进行排序（不包括static和transient修饰的member），没有@Version注解的Field默认version=0；如果version相同，按照version, boolean, primitives, conditionals, object references排序
2. 按照排序的Field把Bean的Field逐个写到输出流
3. @Version的版本只能加不能减小，如果相等的话，有可能因为默认的排序规则，导致流中的Filed顺序和内存中的FSTFieldInfo[]数组的顺序不一致，而注入错误

反序列化：

1. 反序列化按照对象流的格式进行解析，对象流中保存的Field顺序和内存中的FSTFieldInfo顺序保持一致
2. 相同版本的Field在对象流中存在，在内存Bean中缺失：可能抛异常（会有后向兼容问题）
3. 对象流中包含内存Bean中没有的高版本Field：正常（老版本兼容新）
4. 相同版本的Field在对象流中缺失，在内存Bean中存在：抛出异常
5. 相同的Field在对象流和内存Bean中的版本不一致：抛出异常
6. 内存Bean增加了不高于最大版本的Field：抛出异常

01:18:56.442 [main] WARN Serializer-Tools - Serializer-Tools-Deserializer:use FST
unable to decode:
com.fst.FSTBean÷
üv0str
üv1str
01:18:56.455 [main] ERROR Serializer-Tools - error in fstSerializer
java.io.IOException: java.lang.NullPointerException
	at org.nustaq.serialization.FSTObjectInput.readObject(FSTObjectInput.java:247)
	at org.nustaq.serialization.FSTConfiguration.asObject(FSTConfiguration.java:1150)
	at com.taobao.wireless.ripple.common.serialize.FstSerializer.deserialize(FstSerializer.java:27)
	at com.fst.FSTSerial.deserilize(FSTSerial.java:44)
	at com.fst.FSTSerial.main(FSTSerial.java:56)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.lang.NullPointerException: null
	at org.nustaq.serialization.FSTObjectInput.readObjectWithHeader(FSTObjectInput.java:357)
	at org.nustaq.serialization.FSTObjectInput.readObjectFields(FSTObjectInput.java:712)
	at org.nustaq.serialization.FSTObjectInput.instantiateAndReadNoSer(FSTObjectInput.java:566)
	at org.nustaq.serialization.FSTObjectInput.readObjectWithHeader(FSTObjectInput.java:374)
	at org.nustaq.serialization.FSTObjectInput.readObjectInternal(FSTObjectInput.java:331)
	at org.nustaq.serialization.FSTObjectInput.readObject(FSTObjectInput.java:311)
	at org.nustaq.serialization.FSTObjectInput.readObject(FSTObjectInput.java:245)
	... 9 common frames omitted

所以从上面的代码逻辑就可以分析出这个使用规则：@Version的使用原则就是，每新增一个Field，就对应的加上@Version注解，并且把version的值设置为当前版本的最大值加一，不允许删除Field

另外再看一下@Version注解的注释：明确说明了用于后向兼容

package org.nustaq.serialization.annotations;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD})

/**
 * support for adding fields without breaking compatibility to old streams.
 * For each release of your app increment the version value. No Version annotation means version=0.
 * Note that each added field needs to be annotated.
 *
 * e.g.
 *
 * class MyClass implements Serializable {
 *
 *     // fields on initial release 1.0
 *     int x;
 *     String y;
 *
 *     // fields added with release 1.5
 *     @Version(1) String added;
 *     @Version(1) String alsoAdded;
 *
 *     // fields added with release 2.0
 *     @Version(2) String addedv2;
 *     @Version(2) String alsoAddedv2;
 *
 * }
 *
 * If an old class is read, new fields will be set to default values. You can register a VersionConflictListener
 * at FSTObjectInput in order to fill in defaults for new fields.
 *
 * Notes/Limits:
 * - Removing fields will break backward compatibility. You can only Add new fields.
 * - Can slow down serialization over time (if many versions)
 * - does not work for Externalizable or Classes which make use of JDK-special features such as readObject/writeObject
 *   (AKA does not work if fst has to fall back to 'compatible mode' for an object).
 * - in case you use custom serializers, your custom serializer has to handle versioning
 *
 */
public @interface Version {
    byte value();
}

实例验证

下面根据上面分析的结果，一个个验证场景，首先定义一个简单的测试Bean

public class FSTBean implements Serializable {
    /** serialVersionUID */
    private static final long serialVersionUID = -2708653783151699375L;
    
    private Integer v0int
    private String v0str;
}

准备序列化和反序列化方法

public class FSTSerial {

    private static void serialize(FstSerializer fst, String fileName) {
        try {
            FSTBean fstBean = new FSTBean();
            fstBean.setV0int(1);
            fstBean.setV0str("v0str");
            byte[] v1 = fst.serialize(fstBean);

            FileOutputStream fos = new FileOutputStream(new File("byte.bin"));
            fos.write(v1, 0, v1.length);
            fos.close();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static void deserilize(FstSerializer fst, String fileName) {
        try {
            FileInputStream fis = new FileInputStream(new File("byte.bin"));
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            byte[] buf = new byte[256];
            int length = 0;
            while ((length = fis.read(buf)) > 0) {
                baos.write(buf, 0, length);
            }
            fis.close();
            buf = baos.toByteArray();
            FSTBean deserial = fst.deserialize(buf, FSTBean.class);
            System.out.println(deserial);
            System.out.println(deserial);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        FstSerializer fst = new FstSerializer();
        serialize(fst, "byte.bin");
        deserilize(fst, "byte.bin");
    }
}

验证序列化和反序列话没有问题

23:42:57.705 [main] WARN Serializer-Tools - Serializer-Tools:use FST
23:42:57.715 [main] WARN Serializer-Tools - Serializer-Tools-Deserializer:use FST
FSTBean(v0int=1, v0str=v0str)
FSTBean(v0int=1, v0str=v0str)

Case1：单纯读入bin文件，并且向增加一个FSTBean中增加一个成员v1str，抛异常。因为不允许增加相同版本的Field

public class FSTBean implements Serializable {
    /** serialVersionUID */
    private static final long serialVersionUID = -2708653783151699375L;
    
    private Integer v0int
    private String v0str;
    private String v1str;
}

unable to decode:
com.fst.FSTBean÷
üv0str
01:27:29.878 [main] ERROR Serializer-Tools - error in fstSerializer
java.io.IOException: java.lang.ArrayIndexOutOfBoundsException: 28
	at org.nustaq.serialization.FSTObjectInput.readObject(FSTObjectInput.java:247)
	at org.nustaq.serialization.FSTConfiguration.asObject(FSTConfiguration.java:1150)
	at com.taobao.wireless.ripple.common.serialize.FstSerializer.deserialize(FstSerializer.java:27)
	at com.fst.FSTSerial.deserilize(FSTSerial.java:44)
	at com.fst.FSTSerial.main(FSTSerial.java:54)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.lang.ArrayIndexOutOfBoundsException: 28
	at org.nustaq.serialization.coders.FSTStreamDecoder.readFByte(FSTStreamDecoder.java:296)
	at org.nustaq.serialization.coders.FSTStreamDecoder.readFShort(FSTStreamDecoder.java:366)
	at org.nustaq.serialization.FSTClazzNameRegistry.decodeClass(FSTClazzNameRegistry.java:163)
	at org.nustaq.serialization.coders.FSTStreamDecoder.readClass(FSTStreamDecoder.java:478)
	at org.nustaq.serialization.FSTObjectInput.readClass(FSTObjectInput.java:938)
	at org.nustaq.serialization.FSTObjectInput.readObjectWithHeader(FSTObjectInput.java:347)
	at org.nustaq.serialization.FSTObjectInput.readObjectFields(FSTObjectInput.java:712)
	at org.nustaq.serialization.FSTObjectInput.instantiateAndReadNoSer(FSTObjectInput.java:566)
	at org.nustaq.serialization.FSTObjectInput.readObjectWithHeader(FSTObjectInput.java:374)
	at org.nustaq.serialization.FSTObjectInput.readObjectInternal(FSTObjectInput.java:331)
	at org.nustaq.serialization.FSTObjectInput.readObject(FSTObjectInput.java:311)
	at org.nustaq.serialization.FSTObjectInput.readObject(FSTObjectInput.java:245)
	... 9 common frames omitted

Case2：单纯读入bin文件，并且向增加一个FSTBean中增加一个成员v1str，并将增加注解@Version，结果正常，和期望的一致。为了验证不加Version的时候默认version值是0，可以把1变成0再测试，一定会抛异常。

public class FSTBean implements Serializable {
    /** serialVersionUID */
    private static final long serialVersionUID = -2708653783151699375L;
    
    private Integer v0int
    private String v0str;
    @Verison(1)
    private String v1str;
}

01:29:35.678 [main] WARN Serializer-Tools - Serializer-Tools-Deserializer:use FST
FSTBean(v0int=1, v0str=v0str, v1str=null)

Process finished with exit code 0

Case3：单纯读入bin文件，并且现有的v0str增加@Version(1)注解，结果抛异常，说明Field的版本一旦添加后就不允许修改，否则有后向兼容问题。注意看这个异常，对比一下上面提到的 nextVersion != subInfo.getVersion()校验就很清楚了。

public class FSTBean implements Serializable {
    /** serialVersionUID */
    private static final long serialVersionUID = -2708653783151699375L;
    
    private Integer v0int
    @Version(1)
    private String v0str;
}

01:32:58.140 [main] WARN Serializer-Tools - Serializer-Tools-Deserializer:use FST
unable to decode:
com.fst.FSTBean÷
üv0str
01:32:58.157 [main] ERROR Serializer-Tools - error in fstSerializer
java.io.IOException: java.lang.RuntimeException: read version tag -4 fieldInfo has 1
	at org.nustaq.serialization.FSTObjectInput.readObject(FSTObjectInput.java:247)
	at org.nustaq.serialization.FSTConfiguration.asObject(FSTConfiguration.java:1150)
	at com.taobao.wireless.ripple.common.serialize.FstSerializer.deserialize(FstSerializer.java:27)
	at com.fst.FSTSerial.deserilize(FSTSerial.java:44)
	at com.fst.FSTSerial.main(FSTSerial.java:54)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.lang.RuntimeException: read version tag -4 fieldInfo has 1
	at org.nustaq.serialization.FSTObjectInput.readObjectFields(FSTObjectInput.java:674)
	at org.nustaq.serialization.FSTObjectInput.instantiateAndReadNoSer(FSTObjectInput.java:566)
	at org.nustaq.serialization.FSTObjectInput.readObjectWithHeader(FSTObjectInput.java:374)
	at org.nustaq.serialization.FSTObjectInput.readObjectInternal(FSTObjectInput.java:331)
	at org.nustaq.serialization.FSTObjectInput.readObject(FSTObjectInput.java:311)
	at org.nustaq.serialization.FSTObjectInput.readObject(FSTObjectInput.java:245)
	... 9 common frames omitted
null

Process finished with exit code 0

Case4：如果Version间有空隙，比如对象流中定一个了最高版本那是10，想在内存Bean中新定义一个version=5的Field，根据排序的规则，还是会抛异常，所以版本一定要一直增加，不可回退

public class FSTBean implements Serializable {
    /** serialVersionUID */
    private static final long serialVersionUID = -2708653783151699375L;
    
    private Integer v0int
    @Version(10)
    private String v0str;
    @Version(5)  // 版本不可回退！！
    private String v0str;
}

Case5：把已有的v0str删除，没有问题，但是把已有的v0int删除就有问题了，因为涉及到了排序，所以不要对删除字段抱有幻想，依靠不稳定的排序做兼容迟早要出事。

public class FSTBean implements Serializable {
    /** serialVersionUID */
    private static final long serialVersionUID = -2708653783151699375L;
   
    private Integer v0int
    private String v0str;
}

删除v0str正常
01:40:00.548 [main] WARN Serializer-Tools - Serializer-Tools-Deserializer:use FST
FSTBean(v0int=1)

删除v0int异常
01:38:39.479 [main] WARN Serializer-Tools - Serializer-Tools-Deserializer:use FST
java.lang.NullPointerException
	at java.lang.String.length(String.java:623)
	at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:420)
	at java.lang.StringBuilder.append(StringBuilder.java:136)
	at com.fst.FSTBean.toString(FSTBean.java:12)
	at java.lang.String.valueOf(String.java:2994)
	at java.io.PrintStream.println(PrintStream.java:821)
	at com.fst.FSTSerial.deserilize(FSTSerial.java:45)
	at com.fst.FSTSerial.main(FSTSerial.java:54)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

差不多这么多例子了，为啥看这个，因为多租户集群发布过程，出现了新老代码互相消费消息的情况，而消息体Bean的定义是在SDK里，版本升级后没发现，预发环境单台机器不存在兼容问题，所以看不出来，上线到集群环境才发现这个问题。