1.布局文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp"
    android:orientation="vertical"
    tools:context=".MainActivity">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <Button
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:id="@+id/bt_encode"
            android:text="Encode Image"
            android:layout_marginEnd="4dp"/>
        <Button
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:id="@+id/bt_decode"
            android:text="Decode Image"
            android:layout_marginEnd="4dp"/>

    </LinearLayout>
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/text_view"
        android:maxLines="5"
        android:ellipsize="end"
        android:layout_marginTop="8dp"/>
    <ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/image_view"
        android:scaleType="centerCrop"
        android:adjustViewBounds="true"
        android:layout_marginTop="8dp"/>

</LinearLayout>

2.java代码

package com.example.image2base64;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;

import android.Manifest;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.net.Uri;
import android.os.Bundle;
import android.provider.MediaStore;
import android.util.Base64;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import java.io.ByteArrayOutputStream;
import java.io.IOException;

public class MainActivity extends AppCompatActivity {
    Button btEncode,btDecode;
    TextView textView;
    ImageView imageView;
    String sImage;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btEncode = findViewById(R.id.bt_encode);
        btDecode = findViewById(R.id.bt_decode);
        textView = findViewById(R.id.text_view);
        imageView = findViewById(R.id.image_view);
        btEncode.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.READ_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED){
                    ActivityCompat.requestPermissions(MainActivity.this,new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},100);
                }else{
                    selectImage();
                }
            }
        });
        btDecode.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                byte[] bytes = Base64.decode(sImage,Base64.DEFAULT);
                Bitmap bitmap = BitmapFactory.decodeByteArray(bytes,0,bytes.length);
                imageView.setImageBitmap(bitmap);
            }
        });
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if(requestCode == 100 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
            selectImage();
        }else{
            Toast.makeText(this, "权限关闭", Toast.LENGTH_SHORT).show();
        }
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if(requestCode ==100 && resultCode == RESULT_OK && data!=null){
            Uri uri = data.getData();
            try {
                Bitmap bitmap = MediaStore.Images.Media.getBitmap(getContentResolver(),uri);
                ByteArrayOutputStream stream = new ByteArrayOutputStream();
                bitmap.compress(Bitmap.CompressFormat.JPEG,100,stream);
                byte[] bytes = stream.toByteArray();
                sImage = Base64.encodeToString(bytes,Base64.DEFAULT);
                textView.setText(sImage);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }

    private void selectImage(){
        textView.setText("");
        imageView.setImageBitmap(null);
        Intent intent = new Intent(Intent.ACTION_PICK);
        intent.setType("image/*");
        startActivityForResult(Intent.createChooser(intent,"Select Image"),100);
    }
}

3.manifest文件

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>