[Doc] add basic type java format doc

docs/protocols/java_object_graph.md

@@ -1,2 +1,64 @@
 # Java Serialization
-Coming soon
+The data are serialized using little endian order overall.
+
+## Basic types
+### boolean
+- size: 1 byte
+- format: 0 for `false`, 1 for `true`
+
+### byte
+- size: 1 byte
+- format: write as pure byte.
+
+### short
+- size: 2 byte
+- byte order: little endian order
+
+### char
+- size: 2 byte
+- byte order: little endian order
+
+### int
+- size: 1~5 byte
+- positive int format: first bit in every byte indicate whether has next byte. if first bit is set i.e. `b & 0x80 == 0x80`, then next byte should be read util first bit is unset.
+- Negative number will be converted to positive number by ` (v << 1) ^ (v >> 31)` to reduce cost of small negative numbers.
+
+### long
+- size: 1~9 byte
+- positive long format: first bit in every byte indicate whether has next byte. if first bit is set i.e. `b & 0x80 == 0x80`, then next byte should be read util first bit is unset.
+- Negative number will be converted to positive number by ` (v << 1) ^ (v >> 63)` to reduce cost of small negative numbers.
+
+### float
+- size: 4 byte
+- format: convert float to 4 bytes int by `Float.floatToRawIntBits`, then write as binary by little endian order.
+
+### double
+- size: 8 byte
+- format: convert double to 8 bytes int by `Double.doubleToRawLongBits`, then write as binary by little endian order.
+
+## String
+Format:
+- one byte for encoding: 0 for `ascii`, 1 for `utf-16`, 2 for `utf-8`.
+- positive varint for encoded string binary length.
+- encoded string binary data based on encoding: `ascii/utf-16/utf-8`.
+
+Which encoding to choose:
+- For JDK8: fury detect `ascii` at runtime, if string is `ascii` string, then use `ascii` encoding, otherwise use `utf-16`.
+- For JDK9+: fury use `coder` in `String` object for encoding, `ascii`/`utf-16` will be used for encoding.
+- If the string is encoded by `utf-8`, then fury will use `utf-8` to decode the data. But currently fury doesn't enable utf-8 encoding by default for java. Cross-language string serialization of fury use `utf-8` by default.
+
+## Collection
+
+
+## Map
+
+
+## Object
+
+
+
+
+
+
+
+