add reader codegen doc

docs/reader_codegen.md

@@ -0,0 +1,125 @@
+# 反序列化CodeGen算法介绍
+fastjson2会使用codegen来优化反序列化的性能，用到的codegen技术包括：
+* ASM 基于内置asm 9.2裁剪版实现的动态字节码生成类
+* Annotation Process Tools(APT)
+
+## 实现算法介绍
+
+我们要将json反序列化为如下的Image类
+```java
+@Data
+public class Image {
+    private int height;
+    private Size size;
+    private String title;
+    private String uri;
+    private int width;
+}
+```
+
+需要生成如下的代码来快速将json中的name和字段关联起来：
+```java
+public final class Image_FASTJOSNReader
+    extends com.alibaba.fastjson2.reader.ObjectReader5 {
+
+    public Object readObject(
+            com.alibaba.fastjson2.JSONReader jsonReader,
+            java.lang.reflect.Type fieldType,
+            Object fieldName,
+            long features
+    ) {
+        Image object = new Image();
+
+        while (!jsonReader.nextIfObjectEnd()) {
+            switch (jsonReader.getRawInt()) {
+                // '"' | ('w' << 8) | ('i' << 16) | ('d' << 24) == 1684633378
+                // 'd' | ('h' << 8) | ('"' << 16) | (':' << 24) == 975333492
+                case 1684633378: // "wid
+                    if (jsonReader.nextIfName4Match5(975333492)) { // th":
+                        object.setWidth(
+                                jsonReader.readInt32Value()
+                        );
+                        continue;
+                    }
+                    break;
+                // '"' | ('h' << 8) | ('e' << 16) | ('i' << 24) == 1768253474
+                // 'g' | ('h' << 8) | ('t' << 16) | ('"' << 24) == 578054247
+                case 1768253474: // "hei
+                    if (jsonReader.nextIfName4Match6(578054247)) { // ght"
+                        object.setHeight(
+                                jsonReader.readInt32Value()
+                        );
+                        continue;
+                    }
+                    break;
+               // '"' | ('u' << 8) | ('r' << 16) | ('i' << 24) == 1769108770
+                case 1769108770: // "uri"
+                    if (jsonReader.nextIfName4Match3()) {
+                        object.setUri(
+                                jsonReader.readString()
+                        );
+                        continue;
+                    }
+                    break;
+                // ...
+                default:
+                    break;
+            }
+            // ....
+        }
+        return object;
+    }
+}
+```
+
+相关方法在JSONReader中的实现
+```java
+class JSONReaderUTF8 implements JSONReader {
+    public final int getRawInt() {
+        if (offset + 3 < bytes.length) {
+            return UNSAFE.getInt(bytes, ARRAY_BYTE_BASE_OFFSET + offset - 1);
+        }
+        return 0;
+    }
+
+    public boolean nextIfName4Match3() {
+        offset += 5;
+
+        if (bytes[offset - 2] != '"' || bytes[offset - 1] != ':') {
+            return false;
+        }
+
+        // ...
+
+        return true;
+    }
+
+    @Override
+    public final boolean nextIfName4Match4(byte c4) {
+        offset += 6;
+        if (bytes[offset - 3] != c4 || bytes[offset - 2] != '"' || bytes[offset - 1] != ':') {
+            return false;
+        }
+        // ...
+        return true;
+    }
+    
+    public boolean nextIfName4Match5(int name1) {
+        offset += 7;
+        if (UNSAFE.getInt(bytes, ARRAY_BYTE_BASE_OFFSET + offset - 4) != name1) {
+            return false;
+        }
+        // ...
+        return true;
+    }
+    
+    public boolean nextIfName4Match6(int name1) {
+        offset += 8;
+        if (UNSAFE.getInt(bytes, ARRAY_BYTE_BASE_OFFSET + offset - 5) != name1 || bytes[offset - 1] != ':') {
+            return false;
+        }
+        // ...
+        return true;
+    }
+}
+```