篇文章主要列舉了第三人稱的多種控制方式。

一、官方實例的第三人稱控制方式。

該控制方式比較復雜，但是卻寫得很好很完善，并且運用了新的動畫系統。大家可以下載官方的角色控制包來使用，附上圖一張，不多說。

二、老版官方的第三人稱控制方式。

大家應該知道老版的第三人稱控制方式是用JavaScript腳本寫的，可能大家拿過來還不太好用，但是這里我們把它改寫成C#腳本（PS：參照雨松的修改），這樣用起來就方便多了，而且用的是經典版的動畫系統，滿足了很多人的需求。

在unity中，新版的mecanim動畫系統出現，雖然說很實用，在某些方面解決了很多人的需求，但這并不意味著可以替代原版經典的動畫系統，所以到現在為止，兩種動畫都是通用的。

using UnityEngine;

using System.Collections;

[RequireComponent(typeof(CharacterController))]

public class ThirdPersonController111 : MonoBehaviour

{

public AnimationClip idleAnimation;

public AnimationClip walkAnimation;

public AnimationClip runAnimation;

public AnimationClip jumpPoseAnimation;

public float walkMaxAnimationSpeed=0.75f;

public float trotMaxAnimationSpeed=1.0f;

public float runMaxAnimationSpeed=1.0f;

public float jumpAnimationSpeed=1.15f;

public float landAnimationSpeed=1.0f;

private Animation _animation;

enum CharacterState

{

Idle=0,

Walking=1,

Trotting=2,

Running=3,

Jumping=4,

}

private CharacterState _characterState;

// The speed when walking

float walkSpeed=2.0f;

// after trotAfterSeconds of walking we trot with trotSpeed

float trotSpeed=4.0f;

// when pressing "Fire3" button (cmd) we start running

float runSpeed=6.0f;

float inAirControlAcceleration=3.0f;

// How high do we jump when pressing jump and letting go immediately

float jumpHeight=0.5f;

// The gravity for the character

float gravity=20.0f;

// The gravity in controlled descent mode

float speedSmoothing=10.0f;

float rotateSpeed=500.0f;

float trotAfterSeconds=3.0f;

bool canJump=true;

private float jumpRepeatTime=0.05f;

private float jumpTimeout=0.15f;

private float groundedTimeout=0.25f;

// The camera doesnt start following the target immediately but waits for a split second to avoid too much waving around.

private float lockCameraTimer=0.0f;

// The current move direction in x-z

private Vector3 moveDirection=Vector3.zero;

// The current vertical speed

private float verticalSpeed=0.0f;

// The current x-z move speed

private float moveSpeed=0.0f;

// The last collision flags returned from controller.Move

private CollisionFlags collisionFlags;

// Are we jumping? (Initiated with jump button and not grounded yet)

private bool jumping=false;

private bool jumpingReachedApex=false;

// Are we moving backwards (This locks the camera to not do a 180 degree spin)

private bool movingBack=false;

// Is the user pressing any keys?

private bool isMoving=false;

// When did the user start walking (Used for going into trot after a while)

private float walkTimeStart=0.0f;

// Last time the jump button was clicked down

private float lastJumpButtonTime=-10.0f;

// Last time we performed a jump

private float lastJumpTime=-1.0f;

// the height we jumped from (Used to determine for how long to apply extra jump power after jumping.)

private float lastJumpStartHeight=0.0f;

private Vector3 inAirVelocity=Vector3.zero;

private float lastGroundedTime=0.0f;

private bool isControllable=true;

void Awake()

{

moveDirection=transform.TransformDirection(Vector3.forward);

_animation=GetComponent<Animation>();

if (!_animation)

Debug.Log("The character you would like to control doesn't have animations. Moving her might look weird.");

/*

public var idleAnimation : AnimationClip;

public var walkAnimation : AnimationClip;

public var runAnimation : AnimationClip;

public var jumpPoseAnimation : AnimationClip;

*/

if (!idleAnimation)

{

_animation=null;

Debug.Log("No idle animation found. Turning off animations.");

}

if (!walkAnimation)

{

_animation=null;

Debug.Log("No walk animation found. Turning off animations.");

}

if (!runAnimation)

{

_animation=null;

Debug.Log("No run animation found. Turning off animations.");

}

if (!jumpPoseAnimation && canJump)

{

_animation=null;

Debug.Log("No jump animation found and the character has canJump enabled. Turning off animations.");

}

}

void UpdateSmoothedMovementDirection()

{

Transform cameraTransform=Camera.main.transform;

bool grounded=IsGrounded();

// Forward vector relative to the camera along the x-z plane

Vector3 forward=cameraTransform.TransformDirection(Vector3.forward);

forward.y=0;

forward=forward.normalized;

// Right vector relative to the camera

// Always orthogonal to the forward vector

Vector3 right=new Vector3(forward.z, 0, -forward.x);

float v=Input.GetAxisRaw("Vertical");

float h=Input.GetAxisRaw("Horizontal");

// Are we moving backwards or looking backwards

if (v < -0.2f)

movingBack=true;

else

movingBack=false;

bool wasMoving=isMoving;

isMoving=Mathf.Abs(h) > 0.1f || Mathf.Abs(v) > 0.1f;

// Target direction relative to the camera

Vector3 targetDirection=h * right + v * forward;

// Grounded controls

if (grounded)

{

// Lock camera for short period when transitioning moving & standing still

lockCameraTimer +=Time.deltaTime;

if (isMoving !=wasMoving)

lockCameraTimer=0.0f;

// We store speed and direction seperately,

// so that when the character stands still we still have a valid forward direction

// moveDirection is always normalized, and we only update it if there is user input.

if (targetDirection !=Vector3.zero)

{

// If we are really slow, just snap to the target direction

if (moveSpeed < walkSpeed * 0.9f && grounded)

{

moveDirection=targetDirection.normalized;

}

// Otherwise smoothly turn towards it

else

{

moveDirection=Vector3.RotateTowards(moveDirection, targetDirection, rotateSpeed * Mathf.Deg2Rad * Time.deltaTime, 1000);

moveDirection=moveDirection.normalized;

}

}

// Smooth the speed based on the current target direction

float curSmooth=speedSmoothing * Time.deltaTime;

// Choose target speed

//* We want to support analog input but make sure you cant walk faster diagonally than just forward or sideways

float targetSpeed=Mathf.Min(targetDirection.magnitude, 1.0f);

_characterState=CharacterState.Idle;

// Pick speed modifier

if (Input.GetKey(KeyCode.LeftShift) | Input.GetKey(KeyCode.RightShift))

{

targetSpeed *=runSpeed;

_characterState=CharacterState.Running;

}

else if (Time.time - trotAfterSeconds > walkTimeStart)

{

targetSpeed *=trotSpeed;

_characterState=CharacterState.Trotting;

}

else

{

targetSpeed *=walkSpeed;

_characterState=CharacterState.Walking;

}

moveSpeed=Mathf.Lerp(moveSpeed, targetSpeed, curSmooth);

// Reset walk time start when we slow down

if (moveSpeed < walkSpeed * 0.3f)

walkTimeStart=Time.time;

}

// In air controls

else

{

// Lock camera while in air

if (jumping)

lockCameraTimer=0.0f;

if (isMoving)

inAirVelocity +=targetDirection.normalized * Time.deltaTime * inAirControlAcceleration;

}

}

void ApplyJumping()

{

// Prevent jumping too fast after each other

if (lastJumpTime + jumpRepeatTime > Time.time)

return;

if (IsGrounded())

{

// Jump

// - Only when pressing the button down

// - With a timeout so you can press the button slightly before landing

if (canJump && Time.time < lastJumpButtonTime + jumpTimeout)

{

verticalSpeed=CalculateJumpVerticalSpeed(jumpHeight);

SendMessage("DidJump", SendMessageOptions.DontRequireReceiver);

}

}

}

void ApplyGravity()

{

if (isControllable) // don't move player at all if not controllable.

{

// Apply gravity

bool jumpButton=Input.GetButton("Jump");

// When we reach the apex of the jump we send out a message

if (jumping && !jumpingReachedApex && verticalSpeed <=0.0f)

{

jumpingReachedApex=true;

SendMessage("DidJumpReachApex", SendMessageOptions.DontRequireReceiver);

}

if (IsGrounded())

verticalSpeed=0.0f;

else

verticalSpeed -=gravity * Time.deltaTime;

}

}

float CalculateJumpVerticalSpeed(float targetJumpHeight)

{

// From the jump height and gravity we deduce the upwards speed

// for the character to reach at the apex.

return Mathf.Sqrt(2 * targetJumpHeight * gravity);

}

void DidJump()

{

jumping=true;

jumpingReachedApex=false;

lastJumpTime=Time.time;

lastJumpStartHeight=transform.position.y;

lastJumpButtonTime=-10;

_characterState=CharacterState.Jumping;

}

void Update()

{

if (!isControllable)

{

// kill all inputs if not controllable.

Input.ResetInputAxes();

}

if (Input.GetButtonDown("Jump"))

{

lastJumpButtonTime=Time.time;

}

UpdateSmoothedMovementDirection();

// Apply gravity

// - extra power jump modifies gravity

// - controlledDescent mode modifies gravity

ApplyGravity();

// Apply jumping logic

ApplyJumping();

// Calculate actual motion

Vector3 movement=moveDirection * moveSpeed + new Vector3(0, verticalSpeed, 0) + inAirVelocity;

movement *=Time.deltaTime;

// Move the controller

CharacterController controller=GetComponent<CharacterController>();

collisionFlags=controller.Move(movement);

// ANIMATION sector

if (_animation)

{

if (_characterState==CharacterState.Jumping)

{

if (!jumpingReachedApex)

{

_animation[jumpPoseAnimation.name].speed=jumpAnimationSpeed;

_animation[jumpPoseAnimation.name].wrapMode=WrapMode.ClampForever;

_animation.CrossFade(jumpPoseAnimation.name);

}

else

{

_animation[jumpPoseAnimation.name].speed=-landAnimationSpeed;

_animation[jumpPoseAnimation.name].wrapMode=WrapMode.ClampForever;

_animation.CrossFade(jumpPoseAnimation.name);

}

}

else

{

if (controller.velocity.sqrMagnitude < 0.1f)

{

_animation.CrossFade(idleAnimation.name);

}

else

{

if (_characterState==CharacterState.Running)

{

_animation[runAnimation.name].speed=Mathf.Clamp(controller.velocity.magnitude, 0.0f, runMaxAnimationSpeed);

_animation.CrossFade(runAnimation.name);

}

else if (_characterState==CharacterState.Trotting)

{

_animation[walkAnimation.name].speed=Mathf.Clamp(controller.velocity.magnitude, 0.0f, trotMaxAnimationSpeed);

_animation.CrossFade(walkAnimation.name);

}

else if (_characterState==CharacterState.Walking)

{

_animation[walkAnimation.name].speed=Mathf.Clamp(controller.velocity.magnitude, 0.0f, walkMaxAnimationSpeed);

_animation.CrossFade(walkAnimation.name);

}

}

}

}

// ANIMATION sector

// Set rotation to the move direction

if (IsGrounded())

{

transform.rotation=Quaternion.LookRotation(moveDirection);

}

else

{

Vector3 xzMove=movement;

xzMove.y=0;

if (xzMove.sqrMagnitude > 0.001f)

{

transform.rotation=Quaternion.LookRotation(xzMove);

}

}

// We are in jump mode but just became grounded

if (IsGrounded())

{

lastGroundedTime=Time.time;

inAirVelocity=Vector3.zero;

if (jumping)

{

jumping=false;

SendMessage("DidLand", SendMessageOptions.DontRequireReceiver);

}

}

}

void OnControllerColliderHit(ControllerColliderHit hit)

{

// Debug.DrawRay(hit.point, hit.normal);

if (hit.moveDirection.y > 0.01f)

return;

}

float GetSpeed()

{

return moveSpeed;

}

public bool IsJumping()

{

return jumping;

}

bool IsGrounded()

{

return (collisionFlags & CollisionFlags.CollidedBelow) !=0;

}

Vector3 GetDirection()

{

return moveDirection;

}

public bool IsMovingBackwards()

{

return movingBack;

}

public float GetLockCameraTimer()

{

return lockCameraTimer;

}

bool IsMoving()

{

return Mathf.Abs(Input.GetAxisRaw("Vertical")) + Mathf.Abs(Input.GetAxisRaw("Horizontal")) > 0.5f;

}

bool HasJumpReachedApex()

{

return jumpingReachedApex;

}

bool IsGroundedWithTimeout()

{

return lastGroundedTime + groundedTimeout > Time.time;

}

void Reset()

{

gameObject.tag="Player";

}

}

圖一張：

三、根據需求，自己寫自己需要的控制方式。

在本期訓練營中，主角超級瑪麗我才用了一種比較簡潔的控制方式，因為這種方式已經能夠滿足需求，該種方式就是前后左右移動的方式。該方式不需要添加charactercontroller，只需添加膠囊體就可。（PS：不過該方式有個缺點就是必須朝向固定，也就是只能朝向Z軸正方向）

代碼如下：

using UnityEngine;

using System.Collections;

public class MarioMove : MonoBehaviour

{

public float speed=5.0f;

public static bool isGround;

public static bool IsAllowJump;

[SerializeField]

float m_StationaryTurnSpeed=180;

[SerializeField]

float m_MovingTurnSpeed=360;

float m_ForwardAmount;

float m_TurnAmount;

Vector3 m_GroundNormal;

private Vector3 m_Move;

private Transform m_Cam;

private Vector3 m_CamForward;

// Use this for initialization

void Start()

{

// get the transform of the main camera

if (Camera.main !=null)

{

m_Cam=Camera.main.transform;

}

else

{

Debug.LogWarning(

"Warning: no main camera found. Third person character needs a Camera tagged \"MainCamera\", for camera-relative controls.");

// we use self-relative controls in this case, which probably isn't what the user wants, but hey, we warned them!

}

}

void OnCollisionEnter(Collision collision)

{

//if (collision.collider.tag=="Ground")

if (collision.collider.tag !=null)

{

isGround=true;

IsAllowJump=true;

}

else

{

isGround=false;

}

}

// Update is called once per frame

void Update()

{

float h=Input.GetAxis("Horizontal");

float v=Input.GetAxis("Vertical");

GetComponent<Rigidbody>().MovePosition(transform.position - new Vector3(h, 0, v) * speed * Time.deltaTime);

if (isGround==true && Input.GetButton("Jump"))

{

if (IsAllowJump==true)

{

transform.GetComponentInChildren<Animation>().CrossFade("jump");

GetComponent<Rigidbody>().MovePosition(transform.position - new Vector3(-h * 0.1f, -0.15f, -v * 0.1f));

}

}

else if (Input.GetButtonUp("Jump"))

{

IsAllowJump=false;

}

else

{

if (Input.GetAxis("Vertical") > 0.5f ||

Input.GetAxis("Vertical") < -0.5f ||

Input.GetAxis("Horizontal") > 0.5f ||

Input.GetAxis("Horizontal") < -0.5f)

{

transform.GetComponentInChildren<Animation>().CrossFade("run");

}

else if ((Input.GetAxis("Vertical") > 0.0f && Input.GetAxis("Vertical") < 0.5f) ||

(Input.GetAxis("Vertical") > -0.5f && Input.GetAxis("Vertical") < 0.0f) ||

(Input.GetAxis("Horizontal") > 0.0f && Input.GetAxis("Horizontal") < 0.5f) ||

(Input.GetAxis("Horizontal") < 0.0f && Input.GetAxis("Horizontal") > -0.5f))

{

transform.GetComponentInChildren<Animation>().CrossFade("walk");

}

else

{

transform.GetComponentInChildren<Animation>().CrossFade("idle");

}

}

if (m_Cam !=null)

{

// calculate camera relative direction to move:

m_CamForward=Vector3.Scale(m_Cam.forward, new Vector3(1, 0, 1)).normalized;

m_Move=v * m_CamForward + h * m_Cam.right;

}

else

{

// we use world-relative directions in the case of no main camera

m_Move=v * Vector3.forward + h * Vector3.right;

}

Move(m_Move);

}

public void Move(Vector3 move)

{

// convert the world relative moveInput vector into a local-relative

// turn amount and forward amount required to head in the desired

// direction.

if (move.magnitude > 1f) move.Normalize();

move=transform.InverseTransformDirection(move);

//CheckGroundStatus();

move=Vector3.ProjectOnPlane(move, m_GroundNormal);

m_TurnAmount=Mathf.Atan2(move.x, move.z);

m_ForwardAmount=move.z;

ApplyExtraTurnRotation();

}

void ApplyExtraTurnRotation()

{

// help the character turn faster (this is in addition to root rotation in the animation)

float turnSpeed=Mathf.Lerp(m_StationaryTurnSpeed, m_MovingTurnSpeed, m_ForwardAmount);

transform.Rotate(0, m_TurnAmount * turnSpeed * Time.deltaTime, 0);

}

}

在跳躍的代碼部分，這樣寫的目的是實現了按跳躍鍵的時間長短跳的高度不同，和大家小時候玩的超級瑪麗游戲的感覺很像。

附圖一張：

原文鏈接：http://www.manew.com/thread-98040-1-1.html

本文主要研究一下Java 9的Compact Strings

Compressed Strings(Java 6)

Java 6引入了Compressed Strings，對于one byte per character使用byte[]，對于two bytes per character繼續使用char[]；之前可以使用-XX:+UseCompressedStrings來開啟，不過在java7被廢棄了，然后在java8被移除

Compact Strings(Java 9)

Java 9引入了Compact Strings來取代Java 6的Compressed Strings，它的實現更過徹底，完全使用byte[]來替代char[]，同時新引入了一個字段coder來標識是LATIN1還是UTF16

String

java.base/java/lang/String.java

public final class String
 implements java.io.Serializable, Comparable<String>, CharSequence,
 Constable, ConstantDesc {
?
 /**
 * The value is used for character storage.
 *
 * @implNote This field is trusted by the VM, and is a subject to
 * constant folding if String instance is constant. Overwriting this
 * field after construction will cause problems.
 *
 * Additionally, it is marked with {@link Stable} to trust the contents
 * of the array. No other facility in JDK provides this functionality (yet).
 * {@link Stable} is safe here, because value is never null.
 */
 @Stable
 private final byte[] value;
?
 /**
 * The identifier of the encoding used to encode the bytes in
 * {@code value}. The supported values in this implementation are
 *
 * LATIN1
 * UTF16
 *
 * @implNote This field is trusted by the VM, and is a subject to
 * constant folding if String instance is constant. Overwriting this
 * field after construction will cause problems.
 */
 private final byte coder;
?
 /** Cache the hash code for the string */
 private int hash; // Default to 0
?
 /** use serialVersionUID from JDK 1.0.2 for interoperability */
 private static final long serialVersionUID=-6849794470754667710L;
?
 /**
 * If String compaction is disabled, the bytes in {@code value} are
 * always encoded in UTF16.
 *
 * For methods with several possible implementation paths, when String
 * compaction is disabled, only one code path is taken.
 *
 * The instance field value is generally opaque to optimizing JIT
 * compilers. Therefore, in performance-sensitive place, an explicit
 * check of the static boolean {@code COMPACT_STRINGS} is done first
 * before checking the {@code coder} field since the static boolean
 * {@code COMPACT_STRINGS} would be constant folded away by an
 * optimizing JIT compiler. The idioms for these cases are as follows.
 *
 * For code such as:
 *
 * if (coder==LATIN1) { ... }
 *
 * can be written more optimally as
 *
 * if (coder()==LATIN1) { ... }
 *
 * or:
 *
 * if (COMPACT_STRINGS && coder==LATIN1) { ... }
 *
 * An optimizing JIT compiler can fold the above conditional as:
 *
 * COMPACT_STRINGS==true=> if (coder==LATIN1) { ... }
 * COMPACT_STRINGS==false=> if (false) { ... }
 *
 * @implNote
 * The actual value for this field is injected by JVM. The static
 * initialization block is used to set the value here to communicate
 * that this static final field is not statically foldable, and to
 * avoid any possible circular dependency during vm initialization.
 */
 static final boolean COMPACT_STRINGS;
?
 static {
 COMPACT_STRINGS=true;
 }
?
 /**
 * Class String is special cased within the Serialization Stream Protocol.
 *
 * A String instance is written into an ObjectOutputStream according to
 * <a href="{@docRoot}/../specs/serialization/protocol.html#stream-elements">
 * Object Serialization Specification, Section 6.2, "Stream Elements"</a>
 */
 private static final ObjectStreamField[] serialPersistentFields=new ObjectStreamField[0];
?
 /**
 * Initializes a newly created {@code String} object so that it represents
 * an empty character sequence. Note that use of this constructor is
 * unnecessary since Strings are immutable.
 */
 public String() {
 this.value="".value;
 this.coder="".coder;
 }
?
 //......
?
 public char charAt(int index) {
 if (isLatin1()) {
 return StringLatin1.charAt(value, index);
 } else {
 return StringUTF16.charAt(value, index);
 }
 }
?
 public boolean equals(Object anObject) {
 if (this==anObject) {
 return true;
 }
 if (anObject instanceof String) {
 String aString=(String)anObject;
 if (coder()==aString.coder()) {
 return isLatin1() ? StringLatin1.equals(value, aString.value)
 : StringUTF16.equals(value, aString.value);
 }
 }
 return false;
 }
?
 public int compareTo(String anotherString) {
 byte v1[]=value;
 byte v2[]=anotherString.value;
 if (coder()==anotherString.coder()) {
 return isLatin1() ? StringLatin1.compareTo(v1, v2)
 : StringUTF16.compareTo(v1, v2);
 }
 return isLatin1() ? StringLatin1.compareToUTF16(v1, v2)
 : StringUTF16.compareToLatin1(v1, v2);
 }
?
 public int hashCode() {
 int h=hash;
 if (h==0 && value.length > 0) {
 hash=h=isLatin1() ? StringLatin1.hashCode(value)
 : StringUTF16.hashCode(value);
 }
 return h;
 }
?
 public int indexOf(int ch, int fromIndex) {
 return isLatin1() ? StringLatin1.indexOf(value, ch, fromIndex)
 : StringUTF16.indexOf(value, ch, fromIndex);
 }
?
 public String substring(int beginIndex) {
 if (beginIndex < 0) {
 throw new StringIndexOutOfBoundsException(beginIndex);
 }
 int subLen=length() - beginIndex;
 if (subLen < 0) {
 throw new StringIndexOutOfBoundsException(subLen);
 }
 if (beginIndex==0) {
 return this;
 }
 return isLatin1() ? StringLatin1.newString(value, beginIndex, subLen)
 : StringUTF16.newString(value, beginIndex, subLen);
 }
?
 //......
?
 byte coder() {
 return COMPACT_STRINGS ? coder : UTF16;
 }
?
 byte[] value() {
 return value;
 }
?
 private boolean isLatin1() {
 return COMPACT_STRINGS && coder==LATIN1;
 }
?
 @Native static final byte LATIN1=0;
 @Native static final byte UTF16=1;
?
 //......
}

• COMPACT_STRINGS默認為true，即該特性默認是開啟的
• coder方法判斷COMPACT_STRINGS為true的話，則返回coder值，否則返回UTF16；isLatin1方法判斷COMPACT_STRINGS為true且coder為LATIN1則返回true
• 諸如charAt、equals、hashCode、indexOf、substring等等一系列方法都依賴isLatin1方法來區分對待是StringLatin1還是StringUTF16

StringConcatFactory

實例

public class Java9StringDemo {
?
 public static void main(String[] args){
 String stringLiteral="tom";
 String stringObject=stringLiteral + "cat";
 }
}

• 這段代碼stringObject由變量stringLiteral及cat拼接而來

javap

javac src/main/java/com/example/javac/Java9StringDemo.java
javap -v src/main/java/com/example/javac/Java9StringDemo.class
?
 Last modified 2019年4月7日; size 770 bytes
 MD5 checksum fecfca9c829402c358c4d5cb948004ff
 Compiled from "Java9StringDemo.java"
public class com.example.javac.Java9StringDemo
 minor version: 0
 major version: 56
 flags: (0x0021) ACC_PUBLIC, ACC_SUPER
 this_class: #4 // com/example/javac/Java9StringDemo
 super_class: #5 // java/lang/Object
 interfaces: 0, fields: 0, methods: 2, attributes: 3
Constant pool:
 #1=Methodref #5.#14 // java/lang/Object."<init>":()V
 #2=String #15 // tom
 #3=InvokeDynamic #0:#19 // #0:makeConcatWithConstants:(Ljava/lang/String;)Ljava/lang/String;
 #4=Class #20 // com/example/javac/Java9StringDemo
 #5=Class #21 // java/lang/Object
 #6=Utf8 <init>
 #7=Utf8 ()V
 #8=Utf8 Code
 #9=Utf8 LineNumberTable
 #10=Utf8 main
 #11=Utf8 ([Ljava/lang/String;)V
 #12=Utf8 SourceFile
 #13=Utf8 Java9StringDemo.java
 #14=NameAndType #6:#7 // "<init>":()V
 #15=Utf8 tom
 #16=Utf8 BootstrapMethods
 #17=MethodHandle 6:#22 // REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
 #18=String #23 // \u0001cat
 #19=NameAndType #24:#25 // makeConcatWithConstants:(Ljava/lang/String;)Ljava/lang/String;
 #20=Utf8 com/example/javac/Java9StringDemo
 #21=Utf8 java/lang/Object
 #22=Methodref #26.#27 // java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
 #23=Utf8 \u0001cat
 #24=Utf8 makeConcatWithConstants
 #25=Utf8 (Ljava/lang/String;)Ljava/lang/String;
 #26=Class #28 // java/lang/invoke/StringConcatFactory
 #27=NameAndType #24:#32 // makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
 #28=Utf8 java/lang/invoke/StringConcatFactory
 #29=Class #34 // java/lang/invoke/MethodHandles$Lookup
 #30=Utf8 Lookup
 #31=Utf8 InnerClasses
 #32=Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
 #33=Class #35 // java/lang/invoke/MethodHandles
 #34=Utf8 java/lang/invoke/MethodHandles$Lookup
 #35=Utf8 java/lang/invoke/MethodHandles
{
 public com.example.javac.Java9StringDemo();
 descriptor: ()V
 flags: (0x0001) ACC_PUBLIC
 Code:
 stack=1, locals=1, args_size=1
 0: aload_0
 1: invokespecial #1 // Method java/lang/Object."<init>":()V
 4: return
 LineNumberTable:
 line 8: 0
?
 public static void main(java.lang.String[]);
 descriptor: ([Ljava/lang/String;)V
 flags: (0x0009) ACC_PUBLIC, ACC_STATIC
 Code:
 stack=1, locals=3, args_size=1
 0: ldc #2 // String tom
 2: astore_1
 3: aload_1
 4: invokedynamic #3, 0 // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;)Ljava/lang/String;
 9: astore_2
 10: return
 LineNumberTable:
 line 11: 0
 line 12: 3
 line 13: 10
}
SourceFile: "Java9StringDemo.java"
InnerClasses:
 public static final #30=#29 of #33; // Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
 0: #17 REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
 Method arguments:
 #18 \u0001cat

• javap之后可以看到通過Java 9利用InvokeDynamic調用了StringConcatFactory.makeConcatWithConstants方法進行字符串拼接優化；而Java 8則是通過轉換為StringBuilder來進行優化

StringConcatFactory.makeConcatWithConstants

java.base/java/lang/invoke/StringConcatFactory.java

public final class StringConcatFactory {
 //......
?
 /**
 * Concatenation strategy to use. See {@link Strategy} for possible options.
 * This option is controllable with -Djava.lang.invoke.stringConcat JDK option.
 */
 private static Strategy STRATEGY;
?
 /**
 * Default strategy to use for concatenation.
 */
 private static final Strategy DEFAULT_STRATEGY=Strategy.MH_INLINE_SIZED_EXACT;
?
 private enum Strategy {
 /**
 * Bytecode generator, calling into {@link java.lang.StringBuilder}.
 */
 BC_SB,
?
 /**
 * Bytecode generator, calling into {@link java.lang.StringBuilder};
 * but trying to estimate the required storage.
 */
 BC_SB_SIZED,
?
 /**
 * Bytecode generator, calling into {@link java.lang.StringBuilder};
 * but computing the required storage exactly.
 */
 BC_SB_SIZED_EXACT,
?
 /**
 * MethodHandle-based generator, that in the end calls into {@link java.lang.StringBuilder}.
 * This strategy also tries to estimate the required storage.
 */
 MH_SB_SIZED,
?
 /**
 * MethodHandle-based generator, that in the end calls into {@link java.lang.StringBuilder}.
 * This strategy also estimate the required storage exactly.
 */
 MH_SB_SIZED_EXACT,
?
 /**
 * MethodHandle-based generator, that constructs its own byte[] array from
 * the arguments. It computes the required storage exactly.
 */
 MH_INLINE_SIZED_EXACT
 }
?
 static {
 // In case we need to double-back onto the StringConcatFactory during this
 // static initialization, make sure we have the reasonable defaults to complete
 // the static initialization properly. After that, actual users would use
 // the proper values we have read from the properties.
 STRATEGY=DEFAULT_STRATEGY;
 // CACHE_ENABLE=false; // implied
 // CACHE=null; // implied
 // DEBUG=false; // implied
 // DUMPER=null; // implied
?
 Properties props=GetPropertyAction.privilegedGetProperties();
 final String strategy=props.getProperty("java.lang.invoke.stringConcat");
 CACHE_ENABLE=Boolean.parseBoolean(
 props.getProperty("java.lang.invoke.stringConcat.cache"));
 DEBUG=Boolean.parseBoolean(
 props.getProperty("java.lang.invoke.stringConcat.debug"));
 final String dumpPath=props.getProperty("java.lang.invoke.stringConcat.dumpClasses");
?
 STRATEGY=(strategy==null) ? DEFAULT_STRATEGY : Strategy.valueOf(strategy);
 CACHE=CACHE_ENABLE ? new ConcurrentHashMap<>() : null;
 DUMPER=(dumpPath==null) ? null : ProxyClassesDumper.getInstance(dumpPath);
 }
?
 public static CallSite makeConcatWithConstants(MethodHandles.Lookup lookup,
 String name,
 MethodType concatType,
 String recipe,
 Object... constants) throws StringConcatException {
 if (DEBUG) {
 System.out.println("StringConcatFactory " + STRATEGY + " is here for " + concatType + ", {" + recipe + "}, " + Arrays.toString(constants));
 }
?
 return doStringConcat(lookup, name, concatType, false, recipe, constants);
 }
?
 private static CallSite doStringConcat(MethodHandles.Lookup lookup,
 String name,
 MethodType concatType,
 boolean generateRecipe,
 String recipe,
 Object... constants) throws StringConcatException {
 Objects.requireNonNull(lookup, "Lookup is null");
 Objects.requireNonNull(name, "Name is null");
 Objects.requireNonNull(concatType, "Concat type is null");
 Objects.requireNonNull(constants, "Constants are null");
?
 for (Object o : constants) {
 Objects.requireNonNull(o, "Cannot accept null constants");
 }
?
 if ((lookup.lookupModes() & MethodHandles.Lookup.PRIVATE)==0) {
 throw new StringConcatException("Invalid caller: " +
 lookup.lookupClass().getName());
 }
?
 int cCount=0;
 int oCount=0;
 if (generateRecipe) {
 // Mock the recipe to reuse the concat generator code
 char[] value=new char[concatType.parameterCount()];
 Arrays.fill(value, TAG_ARG);
 recipe=new String(value);
 oCount=concatType.parameterCount();
 } else {
 Objects.requireNonNull(recipe, "Recipe is null");
?
 for (int i=0; i < recipe.length(); i++) {
 char c=recipe.charAt(i);
 if (c==TAG_CONST) cCount++;
 if (c==TAG_ARG) oCount++;
 }
 }
?
 if (oCount !=concatType.parameterCount()) {
 throw new StringConcatException(
 "Mismatched number of concat arguments: recipe wants " +
 oCount +
 " arguments, but signature provides " +
 concatType.parameterCount());
 }
?
 if (cCount !=constants.length) {
 throw new StringConcatException(
 "Mismatched number of concat constants: recipe wants " +
 cCount +
 " constants, but only " +
 constants.length +
 " are passed");
 }
?
 if (!concatType.returnType().isAssignableFrom(String.class)) {
 throw new StringConcatException(
 "The return type should be compatible with String, but it is " +
 concatType.returnType());
 }
?
 if (concatType.parameterSlotCount() > MAX_INDY_CONCAT_ARG_SLOTS) {
 throw new StringConcatException("Too many concat argument slots: " +
 concatType.parameterSlotCount() +
 ", can only accept " +
 MAX_INDY_CONCAT_ARG_SLOTS);
 }
?
 String className=getClassName(lookup.lookupClass());
 MethodType mt=adaptType(concatType);
 Recipe rec=new Recipe(recipe, constants);
?
 MethodHandle mh;
 if (CACHE_ENABLE) {
 Key key=new Key(className, mt, rec);
 mh=CACHE.get(key);
 if (mh==null) {
 mh=generate(lookup, className, mt, rec);
 CACHE.put(key, mh);
 }
 } else {
 mh=generate(lookup, className, mt, rec);
 }
 return new ConstantCallSite(mh.asType(concatType));
 }
?
 private static MethodHandle generate(Lookup lookup, String className, MethodType mt, Recipe recipe) throws StringConcatException {
 try {
 switch (STRATEGY) {
 case BC_SB:
 return BytecodeStringBuilderStrategy.generate(lookup, className, mt, recipe, Mode.DEFAULT);
 case BC_SB_SIZED:
 return BytecodeStringBuilderStrategy.generate(lookup, className, mt, recipe, Mode.SIZED);
 case BC_SB_SIZED_EXACT:
 return BytecodeStringBuilderStrategy.generate(lookup, className, mt, recipe, Mode.SIZED_EXACT);
 case MH_SB_SIZED:
 return MethodHandleStringBuilderStrategy.generate(mt, recipe, Mode.SIZED);
 case MH_SB_SIZED_EXACT:
 return MethodHandleStringBuilderStrategy.generate(mt, recipe, Mode.SIZED_EXACT);
 case MH_INLINE_SIZED_EXACT:
 return MethodHandleInlineCopyStrategy.generate(mt, recipe);
 default:
 throw new StringConcatException("Concatenation strategy " + STRATEGY + " is not implemented");
 }
 } catch (Error | StringConcatException e) {
 // Pass through any error or existing StringConcatException
 throw e;
 } catch (Throwable t) {
 throw new StringConcatException("Generator failed", t);
 }
 }
?
 //......
}

• makeConcatWithConstants方法內部調用了doStringConcat，而doStringConcat方法則調用了generate方法來生成MethodHandle；generate根據不同的STRATEGY來生成MethodHandle，這些STRATEGY有BC_SB、BC_SB_SIZED、BC_SB_SIZED_EXACT、MH_SB_SIZED、MH_SB_SIZED_EXACT、MH_INLINE_SIZED_EXACT，默認是MH_INLINE_SIZED_EXACT(可以通過-Djava.lang.invoke.stringConcat來改變默認的策略)

小結

• Java 9引入了Compact Strings來取代Java 6的Compressed Strings，它的實現更過徹底，完全使用byte[]來替代char[]，同時新引入了一個字段coder來標識是LATIN1還是UTF16
• isLatin1方法判斷COMPACT_STRINGS為true且coder為LATIN1則返回true；諸如charAt、equals、hashCode、indexOf、substring等等一系列方法都依賴isLatin1方法來區分對待是StringLatin1還是StringUTF16
• Java 9利用InvokeDynamic調用了StringConcatFactory.makeConcatWithConstants方法進行字符串拼接優化，相比于Java 8通過轉換為StringBuilder來進行優化，Java 9提供了多種STRATEGY可供選擇，這些STRATEGY有BC_SB(等價于Java 8的優化方式)、BC_SB_SIZED、BC_SB_SIZED_EXACT、MH_SB_SIZED、MH_SB_SIZED_EXACT、MH_INLINE_SIZED_EXACT，默認是MH_INLINE_SIZED_EXACT(可以通過-Djava.lang.invoke.stringConcat來改變默認的策略)

doc

• String Compaction
• JEP 254: Compact Strings
• Java 9: Compact Strings
• Compact Strings In Java 9
• Java 9 Compact Strings Example
• Evolution of Strings in Java to Compact Strings and Indify String Concatenation